Compiler Warning (levels 2 and 4) C4200

  • Article

nonstandard extension used : zero-sized array in struct/union

Indicates that a structure or union contains an array that has zero size.

Declaration of a zero-sized array is a Microsoft extension. This causes a Level-2 warning when a C++ file is compiled and a Level-4 warning when a C file is compiled. C++ compilation also gives this warning: "Cannot generate copy-ctor or copy-assignment operator when UDT contains a zero-sized array." This example generates warning C4200:

// C4200.cpp
// compile by using: cl /W4 c4200.cpp
struct A {
    int a[0];  // C4200
};
int main() {
}

This non-standard extension is often used to interface code with external data structures that have a variable length. If this scenario applies to your code, you can disable the warning:

Example

// C4200b.cpp
// compile by using: cl /W4 c4200a.cpp
#pragma warning(disable : 4200)
struct A {
    int a[0];
};
int main() {
}