Includes the C Standard library header <stdarg.h> and adds the associated names to the std namespace. Including this header ensures that the names declared using external linkage in the C Standard library header are declared in the std namespace.

#include <cstdarg>

namespace std {
    using va_list = see below;
}

#define va_arg(V, P)
#define va_copy(VDST, VSRC)
#define va_end(V)
#define va_start(V, P)

Header Files Reference
C++ Standard Library Overview
Thread Safety in the C++ Standard Library