compiler-warning-level-4-c4840.md

description	title	ms.date	f1_keywords	helpviewer_keywords
Learn more about: Compiler Warning (level 4) C4840	Compiler Warning (level 4) C4840	09/13/2018	C4840	C4840

Compiler Warning (level 4) C4840

non-portable use of class 'type' as an argument to a variadic function

Remarks

Classes or structs that are passed to a variadic function must be trivially copyable. When passing such objects, the compiler simply makes a bitwise copy and does not call the constructor or destructor.

This warning is available beginning in Visual Studio 2017.

Example

The following sample generates C4840 and shows how to fix it:

// C4840.cpp
// compile by using: cl /EHsc /W4 C4840.cpp
#include <stdio.h>

int main()
{
    struct S {
        S(int i) : i(i) {}
        S(const S& other) : i(other.i) {}
        operator int() { return i; }
    private:
        int i;
    } s(0);

    printf("%i\n", s); // warning C4840 : non-portable use of class 'main::S'
                       // as an argument to a variadic function
    // To correct the error, you can perform a static cast
    // to convert the object before passing it:
    printf("%i\n", static_cast<int>(s));
}

For strings built and managed using CStringW, the provided operator LPCWSTR() should be used to cast a CStringW object to the C-style string pointer expected by the format string:

    CStringW str1;
    CStringW str2;
    // ...
    str1.Format("%s", static_cast<LPCWSTR>(str2));