Compiler Warning (level 3) C4839

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

Remarks

Classes or structs that are passed to a variadic function such as printf 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 example generates C4839:

// C4839.cpp
// compile by using: cl /EHsc /W3 C4839.cpp
#include <atomic>
#include <memory>
#include <stdio.h>

int main()
{
    std::atomic<int> i(0);
    printf("%i\n", i); // error C4839: non-standard use of class 'std::atomic<int>'
                        // as an argument to a variadic function
                        // note: the constructor and destructor will not be called;
                        // a bitwise copy of the class will be passed as the argument
                        // error C2280: 'std::atomic<int>::atomic(const std::atomic<int> &)':
                        // attempting to reference a deleted function
}

To correct the error, you can call a member function that returns a trivially copyable type,

    std::atomic<int> i(0);
    printf("%i\n", i.load());

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

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