Question

I already know of include guards, but here are some issues i’d like to figure out:

Example 1

Foo.h

int SumOfNums(int i, int j);

Foo.cpp

#include "Foo.h"
int SumOfNums(int i, int j){
   return i+j;
}

main.cpp

#include "Foo.h"
#include "Foo.h"

int main(){
    SumOfNumbs(5,10);
}

This will compile and run ok.

Example 2

Foo.h

int SumOfNums(int i, int j);
int i;

Foo.cpp

#include "Foo.h"
int SumOfNums(int i, int j){
   return i+j;
}

main.cpp

#include "Foo.h"

int main(){
    SumOfNumbs(5,10);
}

Redefinition of ‘i’ according to compier.

Example 3

Foo.h

int SumOfNums(int i, int j);
enum FooBar{FOO, BAR};

Foo.cpp

#include "Foo.h"
int SumOfNums(int i, int j){
   return i+j;
}

main.cpp

#include "Foo.h"

int main(){
    SumOfNumbs(5,10);
}

This will compile and run ok.

Example 4

Foo.h

int SumOfNums(int i, int j);
enum FooBar{FOO, BAR};

Foo.cpp

#include "Foo.h"
int SumOfNums(int i, int j){
   return i+j;
}

main.cpp

#include "Foo.h"
#include "Foo.h"
int main(){
    SumOfNumbs(5,10);
}

Redefinition of FooBar according to compiler.

To sum it up:

Example 1 – Why can Foo.h be included twice in main.cpp, when there’s no include guards?

Example 2 – How does the int variable differ from the function header?

Example 3 – Why doesn’t the linker complain when there’s one definition of FooBar in Foo.cpp and one in main.cpp?

Example 4 – What’s the difference between this and Example 1?

Answer 1

Ex1 – Why can Foo.h be included twice in main.cpp, when there’s no include guards?

Because in that case, it only declares a function. You can have as many declarations as you want.

Ex2 – How does the int variable differ from the function header?

That’s also a definition and thus breaks the one definition rule.

Ex3 – Why doesn’t the linker complain when there’s one definition of FooBar in Foo.cpp and one in main.cpp?

Defining types across translation units is fine.

Ex4 – What’s the difference between this and Ex1 ?

You’re defining the same type twice in the same translation unit – not permitted.