What is an undeclared identifier error and how do I fix it

Question

What are undeclared identifier errors  What are common causes and how do I fix them   Example error texts    For the Visual Studio compiler  error C2065   cout    undeclared identifier For the GCC compiler   cout  undeclared  first use in this function

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Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.

The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.

If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.

Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.

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Every undeclared variable in c error comes because the compiler is not able to find it in the project  One can include the external  header  file of the library in which the variable is defined  Hence in your question  you require  lt stdio h gt   that is a standard input output file  which describes printf    functionality   According to the documentation  the declaration of fprintf   is in  i e  you have to include it  before using the function

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They most often come from forgetting to include the header file that contains the function declaration  for example  this program will give an  undeclared identifier  error   Missing header  int main         std  cout  lt  lt   Hello world    lt  lt  std  endl      return 0      To fix it  we must include the header    include  lt iostream gt  int main         std  cout  lt  lt   Hello world    lt  lt  std  endl      return 0      If you wrote the header and included it correctly  the header may contain the wrong include guard   To read more  see http   msdn microsoft com en-us library aa229215 v vs 60  aspx   Misspelled variable  Another common source of beginner s error occur when you misspelled a variable   int main         int aComplicatedName      AComplicatedName   1      mind the uppercase A        return 0      Incorrect scope  For example  this code would give an error  because you need to use std  string    include  lt string gt   int main         std  string s1    Hello      Correct      string s2    world      WRONG - would give error      Use before declaration  void f     g      void g         g has not been declared before its first use  To fix it  either move the definition of g before f   void g       void f     g        Or add a declaration of g before f   void g       declaration void f     g      void g          definition   stdafx h not on top  VS-specific   This is Visual Studio-specific  In VS  you need to add  include  stdafx h  before any code  Code before it is ignored by the compiler  so if you have this    include  lt iostream gt   include  stdafx h    The  include  lt iostream gt  would be ignored  You need to move it below    include  stdafx h   include  lt iostream gt    Feel free to edit this answer

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one more case where this issue can occur   if a  b  double c  getValue c     here  the value is declared in a condition and then used outside it

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I had the same problem with a custom class  which was defined in a namespace  I tried to use the class without the namespace  causing the compiler error  identifier  MyClass  is undefined   Adding     using namespace  lt MyNamespace gt    or using the class like  MyNamespace  MyClass myClass    solved the problem

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These error meassages  1 For the Visual Studio compiler  error C2065   printf    undeclared identifier 2 For the GCC compiler   printf  undeclared  first use in this function    mean that you use name printf but the compiler does not see where the name was declared and accordingly does not know what it means   Any name used in a program shall be declared before its using  The compiler has to know what the name denotes   In this particular case the compiler does not see the declaration of name printf   As we know  but not the compiler  it is the name of standard C function declared in header  lt stdio h gt  in C or in header  lt cstdio gt  in C   and placed in standard  std    and global       not necessarily  name spaces   So before using this function we have to provide its name declaration to the compiler by including corresponding headers   For example C    include  lt stdio h gt   int main  void        printf   Hello World n         C      include  lt cstdio gt   int main        std  printf   Hello World n           or printf   Hello World n           or   printf   Hello World n         Sometimes the reason of such an error is a simple typo  For example let s assume that you defined function PrintHello  void PrintHello         std  printf   Hello World n         but in main you made a typo and instead of PrintHello you typed printHello with lower case letter  p      include  lt cstdio gt   void PrintHello         std  printf   Hello World n        int main        printHello        In this case the compiler will issue such an error because it does not see the declaration of name printHello   PrintHello and printHello are two different names one of which was declared and other was not declared but used in the body of main

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It happened to me when the auto formatter in a visual studio project sorted my includes after which the pre compiled header was not the first include anymore    In other words  If you have any of these    include  pch h    or   include  lt stdio h gt    or   include  lt iostream gt   include  stdafx h    Put it at the start of your file   If your clang formatter is sorting the files automatically  try putting an enter after the pre compiled header  If it is on IBS Preserve it will sort each  include block separately    include  pch h      must be first   include  bar h      next block  include  baz h   include  foo h    More info at Compiler Error C2065

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It is like Using the function without declaring it   header file will contain the  function printf    Include the header file in your program this is the solution for that  Some user defined functions may also through error when not declared before using it  If  it is used globally no probs

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In C and C   all names have to be declared before they are used  If you try to use the name of a variable or a function that hasn t been declared you will get an  undeclared identifier  error   However  functions are a special case in C  and in C only  in that you don t have to declare them first  The C compiler will the assume the function exists with the number and type of arguments as in the call  If the actual function definition does not match that you will get another error  This special case for functions does not exist in C     You fix these kind of errors by making sure that functions and variables are declared before they are used  In the case of printf you need to include the header file  lt stdio h gt   or  lt cstdio gt  in C      For standard functions  I recommend you check e g  this reference site  and search for the functions you want to use  The documentation for each function tells you what header file you need

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Most of the time  if you are very sure you imported the library in question  Visual Studio will guide you with IntelliSense   Here is what worked for me   Make sure that  include  stdafx h  is declared first  that is  at the top of all of your includes

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A C   identifier is a name used to identify a variable  function  class  module  or any other user-defined item  In C   all names have to be declared before they are used  If you try to use the name of a such that hasn t been declared you will get an  quot undeclared identifier quot  compile-error  According to the documentation  the declaration of printf   is in cstdio i e  you have to include it  before using the function

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Another possible situation  accessing parent  a template class  member in a template class   Fix method  using the parent class member by its full name  by prefixing this- gt  or parentClassName   to the name of the member    see  templates  parent class member variables not visible in inherited class

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Check if you are importing the same packages in your  m and in your  h Example given  I had this very problem with the init method and it was caused by missing the   import   on the  m file

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