Creating an instance of class

Question

What s the difference between lines 1   2   3   4   When do I use each   Why line 3 prints the constructor Foo and line 7 returns an error and line 8 doesn t    include  lt iostream gt       using namespace std   class Foo       public     Foo                cout  lt  lt   constructor Foo n                           class Bar       public     Bar   Foo              cout  lt  lt   constructor Bar n            int main           1    Foo  foo1   new Foo           2    Foo  foo2   new Foo        3    Foo foo3        4    Foo foo4   Foo  Foo           5    Bar  bar1   new Bar    new Foo            6    Bar  bar2   new Bar    new Foo          7    Bar  bar3   new Bar   Foo foo5          8    Bar  bar3   new Bar   Foo  Foo          return 1

User · Answer

Lines 1 2 3 4 will call the default constructor  They are different in the essence as 1 2 are dynamically created object and 3 4 are statically created objects   In Line 7  you create an object inside the argument call  So its an error   And Lines 5 and 6 are invitation for memory leak

User · Answer

Allocates some dynamic memory from the free store  and creates an object in that memory using its default constructor  You never delete it  so the memory is leaked  Does exactly the same as 1  in the case of user-defined types  the parentheses are optional  Allocates some automatic memory  and creates an object in that memory using its default constructor  The memory is released automatically when the object goes out of scope  Similar to 3  Notionally  the named object foo4 is initialised by default-constructing  copying and destroying a temporary object  usually  this is elided giving the same result as 3  Allocates a dynamic object  then initialises a second by copying the first  Both objects are leaked  and there s no way to delete the first since you don t keep a pointer to it  Does exactly the same as 5  Does not compile  Foo foo5 is a declaration  not an expression  function  and constructor  arguments must be expressions  Creates a temporary object  and initialises a dynamic object by copying it  Only the dynamic object is leaked  the temporary is destroyed automatically at the end of the full expression  Note that you can create the temporary with just Foo   rather than the equivalent Foo  Foo    or indeed Foo  Foo  Foo  Foo  Foo         When do I use each     Don t  unless you like unnecessary decorations on your code  When you want to create an object that outlives the current scope  Remember to delete it when you ve finished with it  and learn how to use smart pointers to control the lifetime more conveniently  When you want an object that only exists in the current scope  Don t  unless you think 3 looks boring and what to add some unnecessary decoration  Don t  because it leaks memory with no chance of recovery  Don t  because it leaks memory with no chance of recovery  Don t  because it won t compile When you want to create a dynamic Bar from a temporary Foo

User · Answer

1    Foo  foo1   new Foo       Creates an object of type Foo in dynamic memory  foo1 points to it  Normally  you wouldn t use raw pointers in C    but rather a smart pointer  If Foo was a POD-type  this would perform value-initialization  it doesn t apply here          2    Foo  foo2   new Foo    Identical to before  because Foo is not a POD type         3    Foo foo3    Creates a Foo object called foo3 in automatic storage         4    Foo foo4   Foo  Foo      Uses copy-initialization to create a Foo object called foo4 in automatic storage         5    Bar  bar1   new Bar    new Foo        Uses Bar s conversion constructor to create an object of type Bar in dynamic storage  bar1 is a pointer to it         6    Bar  bar2   new Bar    new Foo      Same as before         7    Bar  bar3   new Bar   Foo foo5      This is just invalid syntax  You can t declare a variable there         8    Bar  bar3   new Bar   Foo  Foo        Would work and work by the same principle to 5 and 6 if bar3 wasn t declared on in 7   5  amp  6 contain memory leaks   Syntax like new Bar   Foo  Foo      is not usual  It s usually new Bar    Foo       - extra parenthesis account for most-vexing parse   corrected

[c++] Creating an instance of class

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