Should a Source File Include Its Own Header File?

In fact, either answer can be considered correct! That's because, in this particular example, they are both reasonable.

Technically, printmoos.cpp does not need #include "printmoos.hpp" because it does not require a forward declaration of the function printMoos. The function definition given in printmoos.cpp also serves as a declaration.

On the other hand, it is a good idea in general for source files to include their own header files.

An Example

To see why a source file might need to include its own header, consider this file:

moobaa.cpp:

in fenced code block

void printMooBaa(int n) {
    printMoo(n);
}

void printMoo(int n) {
    if (n > 0) {
        std::cout << "Moo! " << endl;
        printBaa(n);
    }
}

void printBaa(int n) {
    if (n > 0) {
        std::cout << "Baa! " << endl;
        printMoo(n - 1);
    }
}

Imagine you are the compiler. You are going through this code line by line… what happens?

First, printMooBaa calls printMoo, but printMoo hasn't been declared yet! Then, printMoo calls printBaa, but printBaa also hasn't been declared yet.

The solution: forward declaration! We can add a header file,

moobaa.hpp:

#ifndef MOOBAA_INCLUDED
#define MOOBAA_INCLUDED

void printMooBaa(int n);
void printMoo(int n);
void printBaa(int n);

#endif

and #include our new header file in our code file,

moobaa.cpp:

#include "moobaa.hpp"

void printMooBaa(int n) {
...

Now all our functions have been declared before they're used: our code will compile!

Upshot

In general, it is best practice for a .cpp file to #include its own .hpp file so functions get forward declared.

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