Why importing the header file of class 'A' in the source file of class 'B' makes the content of class 'A' defined inside the header file of class 'B'?

Question

Suppose I have a class called Class:

Class.h:

#pragma once

class Class
{
public:

    std::vector<int> x();
};

Class.cpp:

#include <vector>
#include "Class.h"

std::vector<int> Class::x()
{
    return std::vector<int>();
}

I suppose that this code should not compile since std::vector isn't defined in Class.h. But the code works. It doesn't work if I include the needed headers (in this case it's just vector) after including Class.h. But as long as the needed headers are imported before the inclusion of Class.h, everythig works fine. Also it doesn't work if I try to include Class.h somewhere alone and try to use it. So for example this code won't work:

    #include <iostream>
    #include "Class.h"

    int main()
    {
        Class c;
        auto x = c.x();

        x.push_back(1);
        x.push_back(2);
        x.push_back(3);

        for (auto i : x)
            std::cout << i << ' ';
    }

But if I include the needed headers, It works fine. So in this example, it works if I include the vector header (also has to be before the inclusion of Class.h):

#include <vector>
#include "Class.h"
#include <iostream>

int main()
{
    Class c;
    auto x = c.x();

    x.push_back(1);
    x.push_back(2);
    x.push_back(3);

    for (auto i : x)
        std::cout << i << ' ';
}

I'm thinking of #include as just copy-pasting. In Class.h, no header is being included. How is it possible to say that this function will return an std::vector without it being defined? And why including the headers in the source file makes it as if it was included in the header file?

Answer 1

You do not compile Class.h, you compile the cpp file which #includes Class.h.

If you include the vector header before Class.h in main.cpp, then, after the preprocessor step, your main.cpp file will have the contents of vector header, followed by the contents of Class.h, followed by other code in main.cpp. So the definition of vector will be visible in Class.

If you compile just the Class.h header separately, you will get an error. I advise you always do this to check if your header files have all the necessary headers and can compile without relying on side effects.