Q:

Should I place a POD struct in a .h file or in a .cpp file?

I need to write a few POD structs, but should I place them in a .h or in a .cpp file?
example
struct Vec2
{
    float x, y, z;
}

Should this be placed inside of Vec2.h or inside of Vec2.cpp?

A:

I need to write a few POD structs, but should I place them in a .h or in a .cpp file?

There's nothing in a real POD as you're showing it, that needs to be implemented explicitly in a separate translation unit (.cpp file).

Should this be placed inside of Vec2.h or inside of Vec2.cpp?

Just place 
struct Vec2 {
    float x, y, z;
}; // <<<< note semicolon

in your header file, include it elsewhere and use it. There's no need for Vec2.cpp.

