Nonstandard Behavior

Articolo
01/03/2018

The latest version of this topic can be found at Nonstandard Behavior.

The following sections list some of the places where the Visual C++ implementation of C++ does not comply with the C++ standard. The section numbers given below refer to the section numbers in the C++ 11 standard (ISO/IEC 14882:2011(E)).

The list of compiler limits that differ from those defined in the C++ standard is given in Compiler Limits.

Covariant Return Types

Virtual base classes are not supported as covariant return types when the virtual function has a variable number of arguments. This does not comply with section 10.3, paragraph 7 of the C++ ISO specification. The following sample does not compile, giving compiler error C2688

// CovariantReturn.cpp  
class A   
{  
   virtual A* f(int c, ...);   // remove ...  
};  
  
class B : virtual A  
{  
   B* f(int c, ...);   // C2688 remove ...  
};

Binding Nondependent Names in Templates

The Visual C++ compiler does not currently support binding nondependent names when initially parsing a template. This does not comply with section 14.6.3 of the C++ ISO specification. This can cause overloads declared after the template (but before the template is instantiated) to be seen.

#include <iostream>  
using namespace std;  
  
namespace N {  
   void f(int) { cout << "f(int)" << endl;}  
}  
  
template <class T> void g(T) {  
    N::f('a');   // calls f(char), should call f(int)  
}  
  
namespace N {  
    void f(char) { cout << "f(char)" << endl;}  
}  
  
int main() {  
    g('c');  
}  
// Output: f(char)

Function Exception Specifiers

Function exception specifiers other than throw() are parsed but not used. This does not comply with section 15.4 of the ISO C++ specification. For example:

void f() throw(int); // parsed but not used  
void g() throw();    // parsed and used

For more information on exception specifications, see Exception Specifications.

char_traits::eof()

The C++ standard states that char_traits::eof must not correspond to a valid char_type value. The Visual C++ compiler enforces this constraint for type char, but not for type wchar_t. This does not comply with the requirement in Table 62 in section 12.1.1 of the C++ ISO specification. The example below demonstrates this.

#include <iostream>  
  
int main()  
{  
    using namespace std;  
  
    char_traits<char>::int_type int2 = char_traits<char>::eof();  
    cout << "The eof marker for char_traits<char> is: " << int2 << endl;  
  
    char_traits<wchar_t>::int_type int3 = char_traits<wchar_t>::eof();  
    cout << "The eof marker for char_traits<wchar_t> is: " << int3 << endl;  
}

Storage Location of Objects

The C++ standard (section 1.8 paragraph 6) requires complete C++ objects to have unique storage locations. However with Visual C++, there are cases where types without data members will share a storage location with other types for the lifetime of the object.

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