Is it possible to print out the size of a C++ class at compile-time?

EDITED (3jun2020) This trick works IN ALL C COMPILERS. For Visual C++:

struct X {
    int a,b;
    int c[10];
};
int _tmain(int argc, _TCHAR* argv[])
{
    int dummy;

    switch (dummy) {
    case sizeof(X):
    case sizeof(X):
        break;
    }
    return 0;
}

------ Build started: Project: cpptest, Configuration: Debug Win32 ------ cpptest.cpp c:\work\cpptest\cpptest\cpptest.cpp(29): error C2196: case value '48' already used ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

For other compilers that only print "duplicate case value", see my answer to this question: How can I print the result of sizeof() at compile time in C?

If you really need to to get sizeof(X) in the compiler output, you can use it as a parameter for an incomplete template type:

template<int s> struct Wow;
struct foo {
    int a,b;
};
Wow<sizeof(foo)> wow;

$ g++ -c test.cpp
test.cpp:5: error: aggregate ‘Wow<8> wow’ has incomplete type and cannot be defined

To answer the updated question -- this may be overkill, but it will print out the sizes of your classes at compile time. There is an undocumented command-line switch in the Visual C++ compiler which will display the complete layouts of classes, including their sizes:

That switch is /d1reportSingleClassLayoutXXX, where XXX performs substring matches against the class name.

https://devblogs.microsoft.com/cppblog/diagnosing-hidden-odr-violations-in-visual-c-and-fixing-lnk2022/