compiler-warning-level-4-c4256.md

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Learn more about: Compiler Warning (level 4) C4256	Compiler Warning (level 4) C4256	11/04/2016	C4256	C4256	a755a32e-895a-4837-a2b5-4ea06b736798

Compiler Warning (level 4) C4256

'function' : constructor for class with virtual bases has '...'; calls may not be compatible with older versions of Visual C++

Possible incompatibility.

Consider the following code example. If the definition of the constructor S2::S2( int i, ... ) was compiled by using a version of the Microsoft C++ compiler before version 7, but the following example is compiled by using the current version, the call to the constructor for S3 would not work correctly because of a special-case calling-convention change. If both were compiled by using Visual C++ 6.0, the call would not work quite right either, unless no parameters were passed for the ellipsis.

To fix this warning,

1. Don't use ellipsis in a constructor.

2. Make sure that all components in their project are built with the current version (including any libraries that may define or reference this class), then disable the warning using the warning pragma.

The following sample generates C4256:

// C4256.cpp
// compile with: /W4
// #pragma warning(disable : 4256)
struct S1
{
};

struct S2: virtual public S1
{
   S2( int i, ... )    // C4256
   {
      i = 0;
   }
   /*
   // try the following line instead
   S2( int i)
   {
      i = 0;
   }
   */
};

void func1()
{
   S2 S3( 2, 1, 2 );   // C4256
   // try the following line instead
   // S2 S3( 2 );
}

int main()
{
}