Why can t variables be declared in a switch statement

Question

I ve always wondered this - why can t you declare variables after a case label in a switch statement   In C   you can declare variables pretty much anywhere  and declaring them close to first use is obviously a good thing  but the following still won t work   switch  val        case VAL         This won t work   int newVal   42      break  case ANOTHER VAL            break        The above gives me the following error  MSC       initialization of  newVal  is skipped by  case  label   This seems to be a limitation in other languages too   Why is this such a problem

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I just wanted to emphasize slim s point  A switch construct creates a whole  first-class-citizen scope  So it is posible to declare  and initialize  a variable in a switch statement before the first case label  without an additional bracket pair   switch  val           This  will  work  even in C89      int newVal   42    case VAL    newVal   1984     break  case ANOTHER VAL      newVal   2001    break

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Ok   Just to clarify this strictly has nothing to do with the declaration   It relates only to  quot jumping over the initialization quot   ISO C    03 6 7 3  A lot of the posts here have mentioned that jumping over the declaration may result in the variable  quot not being declared quot    This is not true   An POD object can be declared without an initializer but it will have an indeterminate value    For example  switch  i       case 0       int j      j  has indeterminate value      j   0      j  set  not initialized  to 0  but this statement                is jumped when  i    1       break     case 1         j        j  is in scope here - but it has an indeterminate value      break     Where the object is a non-POD or aggregate the compiler implicitly adds an initializer  and so it is not possible to jump over such a declaration  class A   public    A         switch  i      Error - jumping over initialization of  A       case 0       A j       Compiler implicitly calls default constructor      break     case 1       break     This limitation is not limited to the switch statement   It is also an error to use  goto  to jump over an initialization  goto LABEL        Error jumping over initialization int j   0   LABEL       A bit of trivia is that this is a difference between C   and C   In C  it is not an error to jump over the initialization  As others have mentioned  the solution is to add a nested block so that the lifetime of the variable is limited to the individual case label

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I wrote this answer orginally for this question  However when I finished it I found that answer has been closed  So I posted it here  maybe someone who likes references to standard will find it helpful   Original Code in question   int i  i   2  switch i        case 1           int k          break      case 2          k   1          cout lt  lt k lt  lt endl          break      There are actually 2 questions   1  Why can I declare a variable after case label   It s because in C   label has to be in form   N3337 6 1 1     labeled-statement                   attribute-specifier-seqopt case constant-expression   statement               And in C   declaration statement is also considered as statement  as opposed to C    N3337 6 1      statement                declaration-statement            2  Why can I jump over variable declaration and then use it   Because  N3337 6 7 3     It is possible to transfer into a block  but not in a way that bypasses declarations with initialization  A   program that jumps    The transfer from the condition of a switch statement to a case label is considered a jump in this respect        from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type  class type with a trivial default   constructor and a trivial destructor  a cv-qualified version of one of these types  or an array of one of the   preceding types and is declared without an initializer  8 5     Since k is of scalar type  and is not initialized at point of declaration jumping over it s declaration is possible  This is semantically equivalent   goto label   int x   label  cout  lt  lt  x  lt  lt  endl    However that wouldn t be possible  if x was initialized at point of declaration    goto label       int x   58    error  jumping over declaration with initialization      label      cout  lt  lt  x  lt  lt  endl

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It appears that anonymous objects can be declared or created in a switch case statement for the reason that they cannot be referenced and as such cannot fall through to the next case  Consider this example compiles on GCC 4 5 3 and Visual Studio 2008  might be a compliance issue tho  so experts please weigh in    include  lt cstdlib gt   struct Foo     int main         int i   42       switch  i             case 42          Foo        Apparently valid         break       default          break            return EXIT SUCCESS

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The whole switch statement is in the same scope  To get around it  do this   switch  val        case VAL                   This   will   work         int newVal   42            break       case ANOTHER VAL                break      Note the brackets

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Consider   switch val    case VAL     int newVal   42  default     int newVal   23      In the absence of break statements  sometimes newVal gets declared twice  and you don t know whether it does until runtime  My guess is that the limitation is because of this kind of confusion  What would the scope of newVal be  Convention would dictate that it would be the whole of the switch block  between the braces    I m no C   programmer  but in C   switch val        int x      case VAL          x 1      Works fine  Declaring a variable inside a switch block is fine  Declaring after a case guard is not

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Interesting that this is fine   switch  i        case 0        int j        j   7        break     case 1        break          but this isn t   switch  i        case 0        int j   7        break     case 1        break      I get that a fix is simple enough  but I m not understanding yet why the first example doesn t bother the compiler  As was mentioned earlier  2 years earlier hehe   declaration is not what causes the error  even despite the logic  Initialisation is the problem  If the variable is initialised and declared on the different lines  it compiles

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newVal exists in the entire scope of the switch but is only initialised if the VAL limb is hit  If you create a block around the code in VAL it should be OK

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Case statements are only labels  This means the compiler will interpret this as a jump directly to the label  In C    the problem here is one of scope  Your curly brackets define the scope as everything inside the switch statement  This means that you are left with a scope where a jump will be performed further into the code skipping the initialization    The correct way to handle this is to define a scope specific to that case statement and define your variable within it   switch  val       case VAL           This will work   int newVal   42      break    case ANOTHER VAL        break

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Ok   Just to clarify this strictly has nothing to do with the declaration   It relates only to  quot jumping over the initialization quot   ISO C    03 6 7 3  A lot of the posts here have mentioned that jumping over the declaration may result in the variable  quot not being declared quot    This is not true   An POD object can be declared without an initializer but it will have an indeterminate value    For example  switch  i       case 0       int j      j  has indeterminate value      j   0      j  set  not initialized  to 0  but this statement                is jumped when  i    1       break     case 1         j        j  is in scope here - but it has an indeterminate value      break     Where the object is a non-POD or aggregate the compiler implicitly adds an initializer  and so it is not possible to jump over such a declaration  class A   public    A         switch  i      Error - jumping over initialization of  A       case 0       A j       Compiler implicitly calls default constructor      break     case 1       break     This limitation is not limited to the switch statement   It is also an error to use  goto  to jump over an initialization  goto LABEL        Error jumping over initialization int j   0   LABEL       A bit of trivia is that this is a difference between C   and C   In C  it is not an error to jump over the initialization  As others have mentioned  the solution is to add a nested block so that the lifetime of the variable is limited to the individual case label

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A switch block isn t the same as a succession of if else if blocks  I m surprised no other answer explains it clearly   Consider this switch statement    switch  value        case 1          int a   10          break      case 2          int a   20          break      It may be surprising  but the compiler will not see it as a simple if else if  It will produce the following code    if  value    1      goto label 1  else if  value    2      goto label 2  else     goto label end     label 1      int a   10      goto label end  label 2      int a   20     Already declared       goto label end     label end         The code after the switch block   The case statements are converted into labels and then called with goto  The brackets create a new scope and it is easy to see now why you can t declare two variables with the same name within a switch block   It may look weird  but it is necessary to support fallthrough  that is  not using break to let execution continue to the next case

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newVal exists in the entire scope of the switch but is only initialised if the VAL limb is hit  If you create a block around the code in VAL it should be OK

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C   Standard has  It is possible to transfer into a block  but not in a way that bypasses declarations with initialization  A program that jumps from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type  3 9  and is declared without an initializer  8 5    The code to illustrate this rule    include  lt iostream gt   using namespace std   class X     public      X               cout  lt  lt   constructor   lt  lt  endl             X               cout  lt  lt   destructor   lt  lt  endl            template  lt class type gt  void ill formed       goto lx  ly    type a  lx    goto ly     template  lt class type gt  void ok     ly    type a  lx    goto ly     void test class       ok lt X gt          compile error   ill formed lt X gt        void test scalar        ok lt int gt       ill formed lt int gt        int main int argc  const char  argv         return 0      The code to show the initializer effect    include  lt iostream gt   using namespace std   int test1       int i   0       There jumps fo  case 1  and  case 2    switch i        case 1           Compile error because of the initializer       int r   1         break      case 2        break          void test2       int i   2    switch i        case 1        int r        r  1         break      case 2        cout  lt  lt   r     lt  lt  r  lt  lt  endl        break          int main int argc  const char  argv         test1      test2      return 0

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Try this   switch  val        case VAL                int newVal   42            break

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I wrote this answer orginally for this question  However when I finished it I found that answer has been closed  So I posted it here  maybe someone who likes references to standard will find it helpful   Original Code in question   int i  i   2  switch i        case 1           int k          break      case 2          k   1          cout lt  lt k lt  lt endl          break      There are actually 2 questions   1  Why can I declare a variable after case label   It s because in C   label has to be in form   N3337 6 1 1     labeled-statement                   attribute-specifier-seqopt case constant-expression   statement               And in C   declaration statement is also considered as statement  as opposed to C    N3337 6 1      statement                declaration-statement            2  Why can I jump over variable declaration and then use it   Because  N3337 6 7 3     It is possible to transfer into a block  but not in a way that bypasses declarations with initialization  A   program that jumps    The transfer from the condition of a switch statement to a case label is considered a jump in this respect        from a point where a variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has scalar type  class type with a trivial default   constructor and a trivial destructor  a cv-qualified version of one of these types  or an array of one of the   preceding types and is declared without an initializer  8 5     Since k is of scalar type  and is not initialized at point of declaration jumping over it s declaration is possible  This is semantically equivalent   goto label   int x   label  cout  lt  lt  x  lt  lt  endl    However that wouldn t be possible  if x was initialized at point of declaration    goto label       int x   58    error  jumping over declaration with initialization      label      cout  lt  lt  x  lt  lt  endl

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This question is was originally tagged as  C  and  C    at the same time  The original code is indeed invalid in both C and C    but for completely different unrelated reasons     In C   this code is invalid because the case ANOTHER VAL  label jumps into the scope of variable newVal bypassing its initialization  Jumps that bypass initialization of automatic objects are illegal in C    This side of the issue is correctly addressed by most answers  However  in C language bypassing variable initialization is not an error  Jumping into the scope of a variable over its initialization is legal in C  It simply means that the variable is left uninitialized  The original code does not compile in C for a completely different reason  Label case VAL  in the original code is attached to the declaration of variable newVal  In C language declarations are not statements  They cannot be labeled  And this is what causes the error when this code is interpreted as C code   switch  val        case VAL                  lt - C error is here      int newVal   42      break  case ANOTHER VAL          lt - C   error is here            break       Adding an extra    block fixes both C   and C problems  even though these problems happen to be very different  On the C   side it restricts the scope of newVal  making sure that case ANOTHER VAL  no longer jumps into that scope  which eliminates the C   issue  On the C side that extra    introduces a compound statement  thus making the case VAL  label to apply to a statement  which eliminates the C issue    In C case the problem can be easily solved without the     Just add an empty statement after the case VAL  label and the code will become valid  switch  val        case VAL                 Now it works in C       int newVal   42      break  case ANOTHER VAL            break      Note that even though it is now valid from C point of view  it remains invalid from C   point of view  Symmetrically  in C   case the the problem can be easily solved without the     Just remove the initializer from variable declaration and the code will become valid  switch  val        case VAL     int newVal    newVal   42      break  case ANOTHER VAL         Now it works in C               break      Note that even though it is now valid from C   point of view  it remains invalid from C point of view

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The whole switch statement is in the same scope  To get around it  do this   switch  val        case VAL                   This   will   work         int newVal   42            break       case ANOTHER VAL                break      Note the brackets

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New variables can be decalared only at block scope  You need to write something like this   case VAL         This will work       int newVal   42          break    Of course  newVal only has scope within the braces     Cheers  Ralph

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Interesting that this is fine   switch  i        case 0        int j        j   7        break     case 1        break          but this isn t   switch  i        case 0        int j   7        break     case 1        break      I get that a fix is simple enough  but I m not understanding yet why the first example doesn t bother the compiler  As was mentioned earlier  2 years earlier hehe   declaration is not what causes the error  even despite the logic  Initialisation is the problem  If the variable is initialised and declared on the different lines  it compiles

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Try this   switch  val        case VAL                int newVal   42            break

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I believe the issue at hand is that is the statement was skipped  and you tried to use the var elsewhere  it wouldn t be declared

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After reading all answers and some more research I get a few things   Case statements are only  labels    In C  according to the specification     6 8 1 Labeled Statements   labeled-statement      identifier   statement     case constant-expression   statement     default   statement   In C there isn t any clause that allows for a  labeled declaration   It s just not part of the language   So  case 1  int x 10          printf   x is  d  x   break    This will not compile  see http   codepad org YiyLQTYw  GCC is giving an error   label can only be a part of statement and declaration is not a statement   Even    case 1  int x            x 10              printf   x is  d  x       break    this is also not compiling  see http   codepad org BXnRD3bu  Here I am also getting the same error     In C    according to the specification   labeled-declaration is allowed but labeled -initialization is not allowed   See http   codepad org ZmQ0IyDG     Solution to such condition is two   Either use new scope using     case 1                      int x 10             printf   x is  d   x            break   Or use dummy statement with label  case 1               int x 10             printf   x is  d  x   break   Declare the variable before switch   and initialize it with different values in case statement if it fulfills your requirement  main         int x       Declare before     switch a            case 1  x 10          break       case 2  x 20          break               Some more things with switch statement  Never write any statements in the switch which are not part of any label  because they will never executed   switch a        printf  This will never print       This will never executed      case 1          printf   1            break       default          break      See http   codepad org PA1quYX3

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This question is was originally tagged as  C  and  C    at the same time  The original code is indeed invalid in both C and C    but for completely different unrelated reasons     In C   this code is invalid because the case ANOTHER VAL  label jumps into the scope of variable newVal bypassing its initialization  Jumps that bypass initialization of automatic objects are illegal in C    This side of the issue is correctly addressed by most answers  However  in C language bypassing variable initialization is not an error  Jumping into the scope of a variable over its initialization is legal in C  It simply means that the variable is left uninitialized  The original code does not compile in C for a completely different reason  Label case VAL  in the original code is attached to the declaration of variable newVal  In C language declarations are not statements  They cannot be labeled  And this is what causes the error when this code is interpreted as C code   switch  val        case VAL                  lt - C error is here      int newVal   42      break  case ANOTHER VAL          lt - C   error is here            break       Adding an extra    block fixes both C   and C problems  even though these problems happen to be very different  On the C   side it restricts the scope of newVal  making sure that case ANOTHER VAL  no longer jumps into that scope  which eliminates the C   issue  On the C side that extra    introduces a compound statement  thus making the case VAL  label to apply to a statement  which eliminates the C issue    In C case the problem can be easily solved without the     Just add an empty statement after the case VAL  label and the code will become valid  switch  val        case VAL                 Now it works in C       int newVal   42      break  case ANOTHER VAL            break      Note that even though it is now valid from C point of view  it remains invalid from C   point of view  Symmetrically  in C   case the the problem can be easily solved without the     Just remove the initializer from variable declaration and the code will become valid  switch  val        case VAL     int newVal    newVal   42      break  case ANOTHER VAL         Now it works in C               break      Note that even though it is now valid from C   point of view  it remains invalid from C point of view

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I just wanted to emphasize slim s point  A switch construct creates a whole  first-class-citizen scope  So it is posible to declare  and initialize  a variable in a switch statement before the first case label  without an additional bracket pair   switch  val           This  will  work  even in C89      int newVal   42    case VAL    newVal   1984     break  case ANOTHER VAL      newVal   2001    break

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It appears that anonymous objects can be declared or created in a switch case statement for the reason that they cannot be referenced and as such cannot fall through to the next case  Consider this example compiles on GCC 4 5 3 and Visual Studio 2008  might be a compliance issue tho  so experts please weigh in    include  lt cstdlib gt   struct Foo     int main         int i   42       switch  i             case 42          Foo        Apparently valid         break       default          break            return EXIT SUCCESS

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C   Standard has  It is possible to transfer into a block  but not in a way that bypasses declarations with initialization  A program that jumps from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type  3 9  and is declared without an initializer  8 5    The code to illustrate this rule    include  lt iostream gt   using namespace std   class X     public      X               cout  lt  lt   constructor   lt  lt  endl             X               cout  lt  lt   destructor   lt  lt  endl            template  lt class type gt  void ill formed       goto lx  ly    type a  lx    goto ly     template  lt class type gt  void ok     ly    type a  lx    goto ly     void test class       ok lt X gt          compile error   ill formed lt X gt        void test scalar        ok lt int gt       ill formed lt int gt        int main int argc  const char  argv         return 0      The code to show the initializer effect    include  lt iostream gt   using namespace std   int test1       int i   0       There jumps fo  case 1  and  case 2    switch i        case 1           Compile error because of the initializer       int r   1         break      case 2        break          void test2       int i   2    switch i        case 1        int r        r  1         break      case 2        cout  lt  lt   r     lt  lt  r  lt  lt  endl        break          int main int argc  const char  argv         test1      test2      return 0

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So far the answers have been for C     For C    you can t jump over an initialization  You can in C  However  in C  a declaration is not a statement  and case labels have to be followed by statements   So  valid  but ugly  C  invalid C    switch  something      case 1      Ugly hack empty statement     int i   6      do stuff with i i       break    case 2      do something        break    default      get a life        Conversly  in C    a declaration is a statement  so the following is valid C    invalid C  switch  something      case 1      do something        break    case 2      int i   12      do something else

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Try this   switch  val        case VAL                int newVal   42            break

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The entire section of the switch is a single declaration context   You can t declare a variable in a case statement like that   Try this instead   switch  val        case VAL         This will work   int newVal   42    break    case ANOTHER VAL            break

User · Answer

New variables can be decalared only at block scope  You need to write something like this   case VAL         This will work       int newVal   42          break    Of course  newVal only has scope within the braces     Cheers  Ralph

User · Answer

Case statements are only labels  This means the compiler will interpret this as a jump directly to the label  In C    the problem here is one of scope  Your curly brackets define the scope as everything inside the switch statement  This means that you are left with a scope where a jump will be performed further into the code skipping the initialization    The correct way to handle this is to define a scope specific to that case statement and define your variable within it   switch  val       case VAL           This will work   int newVal   42      break    case ANOTHER VAL        break

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I just wanted to emphasize slim s point  A switch construct creates a whole  first-class-citizen scope  So it is posible to declare  and initialize  a variable in a switch statement before the first case label  without an additional bracket pair   switch  val           This  will  work  even in C89      int newVal   42    case VAL    newVal   1984     break  case ANOTHER VAL      newVal   2001    break

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You can declare variables within a switch statement if you start a new block   switch  thing       case A          int i   0      Completely legal       break      The reason is to do with allocating  and reclaiming  space on the stack for storage of the local variable s

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newVal exists in the entire scope of the switch but is only initialised if the VAL limb is hit  If you create a block around the code in VAL it should be OK

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Consider   switch val    case VAL     int newVal   42  default     int newVal   23      In the absence of break statements  sometimes newVal gets declared twice  and you don t know whether it does until runtime  My guess is that the limitation is because of this kind of confusion  What would the scope of newVal be  Convention would dictate that it would be the whole of the switch block  between the braces    I m no C   programmer  but in C   switch val        int x      case VAL          x 1      Works fine  Declaring a variable inside a switch block is fine  Declaring after a case guard is not

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I just wanted to emphasize slim s point  A switch construct creates a whole  first-class-citizen scope  So it is posible to declare  and initialize  a variable in a switch statement before the first case label  without an additional bracket pair   switch  val           This  will  work  even in C89      int newVal   42    case VAL    newVal   1984     break  case ANOTHER VAL      newVal   2001    break

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If your code says  int newVal 42  then you would reasonably expect that newVal is never uninitialised  But if you goto over this statement  which is what you re doing  then that s exactly what happens - newVal is in-scope but has not been assigned   If that is what you really meant to happen then the language requires to make it explicit by saying  int newVal  newVal   42    Otherwise you can limit the scope of newVal to the single case  which is more likely what you wanted   It may clarify things if you consider the same example but with  const int newVal   42

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If your code says  int newVal 42  then you would reasonably expect that newVal is never uninitialised  But if you goto over this statement  which is what you re doing  then that s exactly what happens - newVal is in-scope but has not been assigned   If that is what you really meant to happen then the language requires to make it explicit by saying  int newVal  newVal   42    Otherwise you can limit the scope of newVal to the single case  which is more likely what you wanted   It may clarify things if you consider the same example but with  const int newVal   42

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I believe the issue at hand is that is the statement was skipped  and you tried to use the var elsewhere  it wouldn t be declared

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The whole switch statement is in the same scope  To get around it  do this   switch  val        case VAL                   This   will   work         int newVal   42            break       case ANOTHER VAL                break      Note the brackets

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newVal exists in the entire scope of the switch but is only initialised if the VAL limb is hit  If you create a block around the code in VAL it should be OK

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I believe the issue at hand is that is the statement was skipped  and you tried to use the var elsewhere  it wouldn t be declared

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Most of the replies so far are wrong in one respect  you can declare variables after the case statement  but you can t initialize them   case 1      int x     Works     int y   0     Error  initialization is skipped by case     break  case 2            As previously mentioned  a nice way around this is to use braces to create a scope for your case

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New variables can be decalared only at block scope  You need to write something like this   case VAL         This will work       int newVal   42          break    Of course  newVal only has scope within the braces     Cheers  Ralph

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New variables can be decalared only at block scope  You need to write something like this   case VAL         This will work       int newVal   42          break    Of course  newVal only has scope within the braces     Cheers  Ralph

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My favorite evil switch trick is to use an if 0  to skip over an unwanted case label   switch val    case 0     Do something if  0    case 1     Do something else   case 2     Do something in all cases     But very evil

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Ok   Just to clarify this strictly has nothing to do with the declaration   It relates only to  quot jumping over the initialization quot   ISO C    03 6 7 3  A lot of the posts here have mentioned that jumping over the declaration may result in the variable  quot not being declared quot    This is not true   An POD object can be declared without an initializer but it will have an indeterminate value    For example  switch  i       case 0       int j      j  has indeterminate value      j   0      j  set  not initialized  to 0  but this statement                is jumped when  i    1       break     case 1         j        j  is in scope here - but it has an indeterminate value      break     Where the object is a non-POD or aggregate the compiler implicitly adds an initializer  and so it is not possible to jump over such a declaration  class A   public    A         switch  i      Error - jumping over initialization of  A       case 0       A j       Compiler implicitly calls default constructor      break     case 1       break     This limitation is not limited to the switch statement   It is also an error to use  goto  to jump over an initialization  goto LABEL        Error jumping over initialization int j   0   LABEL       A bit of trivia is that this is a difference between C   and C   In C  it is not an error to jump over the initialization  As others have mentioned  the solution is to add a nested block so that the lifetime of the variable is limited to the individual case label

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You can declare variables within a switch statement if you start a new block   switch  thing       case A          int i   0      Completely legal       break      The reason is to do with allocating  and reclaiming  space on the stack for storage of the local variable s

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The entire section of the switch is a single declaration context   You can t declare a variable in a case statement like that   Try this instead   switch  val        case VAL         This will work   int newVal   42    break    case ANOTHER VAL            break

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Consider   switch val    case VAL     int newVal   42  default     int newVal   23      In the absence of break statements  sometimes newVal gets declared twice  and you don t know whether it does until runtime  My guess is that the limitation is because of this kind of confusion  What would the scope of newVal be  Convention would dictate that it would be the whole of the switch block  between the braces    I m no C   programmer  but in C   switch val        int x      case VAL          x 1      Works fine  Declaring a variable inside a switch block is fine  Declaring after a case guard is not

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The entire section of the switch is a single declaration context   You can t declare a variable in a case statement like that   Try this instead   switch  val        case VAL         This will work   int newVal   42    break    case ANOTHER VAL            break

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You can t do this  because case labels are actually just entry points into the containing block   This is most clearly illustrated by Duff s device  Here s some code from Wikipedia   strcpy char  to  char  from  size t count        int n    count   7    8      switch  count   8        case 0  do    to    from        case 7        to    from        case 6        to    from        case 5        to    from        case 4        to    from        case 3        to    from        case 2        to    from        case 1        to    from                     while  --n  gt  0             Notice how the case labels totally ignore the block boundaries  Yes  this is evil  But this is why your code example doesn t work  Jumping to a case label is the same as using goto  so you aren t allowed to jump over a local variable with a constructor   As several other posters have indicated  you need to put in a block of your own   switch             case FOO            MyObject x                           break

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Try this   switch  val        case VAL                int newVal   42            break

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My favorite evil switch trick is to use an if 0  to skip over an unwanted case label   switch val    case 0     Do something if  0    case 1     Do something else   case 2     Do something in all cases     But very evil

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Most of the replies so far are wrong in one respect  you can declare variables after the case statement  but you can t initialize them   case 1      int x     Works     int y   0     Error  initialization is skipped by case     break  case 2            As previously mentioned  a nice way around this is to use braces to create a scope for your case

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After reading all answers and some more research I get a few things   Case statements are only  labels    In C  according to the specification     6 8 1 Labeled Statements   labeled-statement      identifier   statement     case constant-expression   statement     default   statement   In C there isn t any clause that allows for a  labeled declaration   It s just not part of the language   So  case 1  int x 10          printf   x is  d  x   break    This will not compile  see http   codepad org YiyLQTYw  GCC is giving an error   label can only be a part of statement and declaration is not a statement   Even    case 1  int x            x 10              printf   x is  d  x       break    this is also not compiling  see http   codepad org BXnRD3bu  Here I am also getting the same error     In C    according to the specification   labeled-declaration is allowed but labeled -initialization is not allowed   See http   codepad org ZmQ0IyDG     Solution to such condition is two   Either use new scope using     case 1                      int x 10             printf   x is  d   x            break   Or use dummy statement with label  case 1               int x 10             printf   x is  d  x   break   Declare the variable before switch   and initialize it with different values in case statement if it fulfills your requirement  main         int x       Declare before     switch a            case 1  x 10          break       case 2  x 20          break               Some more things with switch statement  Never write any statements in the switch which are not part of any label  because they will never executed   switch a        printf  This will never print       This will never executed      case 1          printf   1            break       default          break      See http   codepad org PA1quYX3

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If your code says  int newVal 42  then you would reasonably expect that newVal is never uninitialised  But if you goto over this statement  which is what you re doing  then that s exactly what happens - newVal is in-scope but has not been assigned   If that is what you really meant to happen then the language requires to make it explicit by saying  int newVal  newVal   42    Otherwise you can limit the scope of newVal to the single case  which is more likely what you wanted   It may clarify things if you consider the same example but with  const int newVal   42

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Ok   Just to clarify this strictly has nothing to do with the declaration   It relates only to  quot jumping over the initialization quot   ISO C    03 6 7 3  A lot of the posts here have mentioned that jumping over the declaration may result in the variable  quot not being declared quot    This is not true   An POD object can be declared without an initializer but it will have an indeterminate value    For example  switch  i       case 0       int j      j  has indeterminate value      j   0      j  set  not initialized  to 0  but this statement                is jumped when  i    1       break     case 1         j        j  is in scope here - but it has an indeterminate value      break     Where the object is a non-POD or aggregate the compiler implicitly adds an initializer  and so it is not possible to jump over such a declaration  class A   public    A         switch  i      Error - jumping over initialization of  A       case 0       A j       Compiler implicitly calls default constructor      break     case 1       break     This limitation is not limited to the switch statement   It is also an error to use  goto  to jump over an initialization  goto LABEL        Error jumping over initialization int j   0   LABEL       A bit of trivia is that this is a difference between C   and C   In C  it is not an error to jump over the initialization  As others have mentioned  the solution is to add a nested block so that the lifetime of the variable is limited to the individual case label

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The entire section of the switch is a single declaration context   You can t declare a variable in a case statement like that   Try this instead   switch  val        case VAL         This will work   int newVal   42    break    case ANOTHER VAL            break

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I believe the issue at hand is that is the statement was skipped  and you tried to use the var elsewhere  it wouldn t be declared

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A switch block isn t the same as a succession of if else if blocks  I m surprised no other answer explains it clearly   Consider this switch statement    switch  value        case 1          int a   10          break      case 2          int a   20          break      It may be surprising  but the compiler will not see it as a simple if else if  It will produce the following code    if  value    1      goto label 1  else if  value    2      goto label 2  else     goto label end     label 1      int a   10      goto label end  label 2      int a   20     Already declared       goto label end     label end         The code after the switch block   The case statements are converted into labels and then called with goto  The brackets create a new scope and it is easy to see now why you can t declare two variables with the same name within a switch block   It may look weird  but it is necessary to support fallthrough  that is  not using break to let execution continue to the next case

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If your code says  int newVal 42  then you would reasonably expect that newVal is never uninitialised  But if you goto over this statement  which is what you re doing  then that s exactly what happens - newVal is in-scope but has not been assigned   If that is what you really meant to happen then the language requires to make it explicit by saying  int newVal  newVal   42    Otherwise you can limit the scope of newVal to the single case  which is more likely what you wanted   It may clarify things if you consider the same example but with  const int newVal   42

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My favorite evil switch trick is to use an if 0  to skip over an unwanted case label   switch val    case 0     Do something if  0    case 1     Do something else   case 2     Do something in all cases     But very evil

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Case statements are only labels  This means the compiler will interpret this as a jump directly to the label  In C    the problem here is one of scope  Your curly brackets define the scope as everything inside the switch statement  This means that you are left with a scope where a jump will be performed further into the code skipping the initialization    The correct way to handle this is to define a scope specific to that case statement and define your variable within it   switch  val       case VAL           This will work   int newVal   42      break    case ANOTHER VAL        break

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You can t do this  because case labels are actually just entry points into the containing block   This is most clearly illustrated by Duff s device  Here s some code from Wikipedia   strcpy char  to  char  from  size t count        int n    count   7    8      switch  count   8        case 0  do    to    from        case 7        to    from        case 6        to    from        case 5        to    from        case 4        to    from        case 3        to    from        case 2        to    from        case 1        to    from                     while  --n  gt  0             Notice how the case labels totally ignore the block boundaries  Yes  this is evil  But this is why your code example doesn t work  Jumping to a case label is the same as using goto  so you aren t allowed to jump over a local variable with a constructor   As several other posters have indicated  you need to put in a block of your own   switch             case FOO            MyObject x                           break

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Consider   switch val    case VAL     int newVal   42  default     int newVal   23      In the absence of break statements  sometimes newVal gets declared twice  and you don t know whether it does until runtime  My guess is that the limitation is because of this kind of confusion  What would the scope of newVal be  Convention would dictate that it would be the whole of the switch block  between the braces    I m no C   programmer  but in C   switch val        int x      case VAL          x 1      Works fine  Declaring a variable inside a switch block is fine  Declaring after a case guard is not

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So far the answers have been for C     For C    you can t jump over an initialization  You can in C  However  in C  a declaration is not a statement  and case labels have to be followed by statements   So  valid  but ugly  C  invalid C    switch  something      case 1      Ugly hack empty statement     int i   6      do stuff with i i       break    case 2      do something        break    default      get a life        Conversly  in C    a declaration is a statement  so the following is valid C    invalid C  switch  something      case 1      do something        break    case 2      int i   12      do something else

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Most of the replies so far are wrong in one respect  you can declare variables after the case statement  but you can t initialize them   case 1      int x     Works     int y   0     Error  initialization is skipped by case     break  case 2            As previously mentioned  a nice way around this is to use braces to create a scope for your case

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My favorite evil switch trick is to use an if 0  to skip over an unwanted case label   switch val    case 0     Do something if  0    case 1     Do something else   case 2     Do something in all cases     But very evil

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Case statements are only labels  This means the compiler will interpret this as a jump directly to the label  In C    the problem here is one of scope  Your curly brackets define the scope as everything inside the switch statement  This means that you are left with a scope where a jump will be performed further into the code skipping the initialization    The correct way to handle this is to define a scope specific to that case statement and define your variable within it   switch  val       case VAL           This will work   int newVal   42      break    case ANOTHER VAL        break

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The whole switch statement is in the same scope  To get around it  do this   switch  val        case VAL                   This   will   work         int newVal   42            break       case ANOTHER VAL                break      Note the brackets

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Most of the replies so far are wrong in one respect  you can declare variables after the case statement  but you can t initialize them   case 1      int x     Works     int y   0     Error  initialization is skipped by case     break  case 2            As previously mentioned  a nice way around this is to use braces to create a scope for your case

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You can declare variables within a switch statement if you start a new block   switch  thing       case A          int i   0      Completely legal       break      The reason is to do with allocating  and reclaiming  space on the stack for storage of the local variable s

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You can t do this  because case labels are actually just entry points into the containing block   This is most clearly illustrated by Duff s device  Here s some code from Wikipedia   strcpy char  to  char  from  size t count        int n    count   7    8      switch  count   8        case 0  do    to    from        case 7        to    from        case 6        to    from        case 5        to    from        case 4        to    from        case 3        to    from        case 2        to    from        case 1        to    from                     while  --n  gt  0             Notice how the case labels totally ignore the block boundaries  Yes  this is evil  But this is why your code example doesn t work  Jumping to a case label is the same as using goto  so you aren t allowed to jump over a local variable with a constructor   As several other posters have indicated  you need to put in a block of your own   switch             case FOO            MyObject x                           break

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You can declare variables within a switch statement if you start a new block   switch  thing       case A          int i   0      Completely legal       break      The reason is to do with allocating  and reclaiming  space on the stack for storage of the local variable s

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You can t do this  because case labels are actually just entry points into the containing block   This is most clearly illustrated by Duff s device  Here s some code from Wikipedia   strcpy char  to  char  from  size t count        int n    count   7    8      switch  count   8        case 0  do    to    from        case 7        to    from        case 6        to    from        case 5        to    from        case 4        to    from        case 3        to    from        case 2        to    from        case 1        to    from                     while  --n  gt  0             Notice how the case labels totally ignore the block boundaries  Yes  this is evil  But this is why your code example doesn t work  Jumping to a case label is the same as using goto  so you aren t allowed to jump over a local variable with a constructor   As several other posters have indicated  you need to put in a block of your own   switch             case FOO            MyObject x                           break

[c++] Why can't variables be declared in a switch statement?

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