Header files and include for sourcing functions

Question

I am quite new to C++, coming from an interpreted language I am having a bit of difficulty with understanding how header files relate to libraries and dependencies.

For example, suppose I have the main body of my project:

#include <vector>

//----------------------------define some auxiliary function which deals with vectors
void A(std::vector<int> &vect)
{ 
    vect.clear();
}

//----------------------------main body
int main()
{
    std::vector<int> vect = {1,2,3}
    A(vect);
    return 0 ;
}

Now suppose I want to define the auxiliary function A() in a separate .cpp file, and still be able to call it from my main .cpp file. My question is- for this project, do I have to #include <vector> in all .cpp files which use something from that library? If so, do I still need to #include <vector> in my main file?

I think I understand how header files work, but it is still unclear what happens if I have several files using elements from the same library.

@MathewHD yes, but do all files which use it have to include it separately? — dvd280
– dvd280, Commented Sep 22, 2020 at 7:18

IndieGameDev · Accepted Answer · 2020-09-22 07:53:33Z

3

Just the file that uses the Vector Component needs to include it. You can also include a component when you include another file.

For Example:

Example.h

#include <vector>

//----------------------------define some auxiliary function which deals with vectors
void A(std::vector<int> &vect) { 
    vect.clear();
}

Example.cp

include "Example.h"

//----------------------------main body
int main() {
    std::vector<int> vect = {1,2,3};
    A(vect);
    return 0;
}

Comment from @Vivick

You would normally also #include <vector> in the main to avoid needing to look up which header file includes which library.

edited Sep 22, 2020 at 7:53

answered Sep 22, 2020 at 7:24

IndieGameDev

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5 Comments

Vivick Over a year ago

Though it is good practice to also include vector in main to avoid looking up which header includes what

463035818_is_not_an_ai Over a year ago

in your example main is using a vector, but does not include the header. If I keep the first line in main but replace Example.h by something else then main is broken. You should include what you use, not rely on includes by other includes

IndieGameDev Over a year ago

@Vivick It's not like OP can't do that. I just wanted to show that you don't have to include <vector> in every script but could use a include on the header file instead.

463035818_is_not_an_ai Over a year ago

what can be done and what should be done are two different things. You should at least mention it

IndieGameDev Over a year ago

It has already been added as a comment, I can include the comment into the answer if wanted tough.

aoiTenshi · Accepted Answer · 2020-09-22 07:25:09Z

2

You don't have to include in all .cpp files. If you #include in your Example.h or Example.cpp and if you just include one of them in your main file like #include "Example.h" or "Example.cpp", you can use vectors in your main too.

answered Sep 22, 2020 at 7:25

aoiTenshi

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Comments

463035818_is_not_an_ai · Accepted Answer · 2020-09-22 07:48:22Z

Includes are transitive. #include foo.h simply puts the contents of foo.h in place of the include-directive.

This is correct, but bad:

// foo.h
#include <vector>
std::vector<int> v;

// bar.h 
#include "foo.h"

// main.cpp
#include "bar.h"   
std::vector<int> x;
int main() {}

This will compile, but it is very bad style. You should include what you use. If someone looks at foo.h only, suppose they realize that v is not needed anymore, they will see that no vector is used, remove #include <vector> and by this break main.cpp. You cannot look into all files that include foo.h to decide whether the include is needed or not.

This is correct and better:

// foo.h
#include <vector>
std::vector<int> v;

// bar.h 
#include "foo.h"

// main.cpp
#include "bar.h"   
#include <vector>
std::vector<int> x;
int main() {}

Some recommend to include standard library headers last. In this way you will get an error in case eg foo.h forgot to include <vector>. Sometimes headers need not the full definition of a type, but only a declaration. In such cases a forward declaration can be used to reduce compile times. The definition is then only supplied in the source by including the header. Example:

// foo.h
struct bar;   // forward declaration 
struct foo {
    bar* p;   // pointer, need only declaration, no definition!
    void do_something();
}

// foo.cpp
#include "bar.h"
void foo::do_something() {
    bar b;      // create instance, needs definition
    // ...
}

PS: You do not have to worry about including a header too often. Headers are guarded against multiple includes via so called include guards:

// moo.h
#ifndef MOO_HEADER_INCLUDE_GUARD
#define MOO_HEADER_INCLUDE_GUARD

// the actual contents

#endif

If in one translation unit the same header is included twice, the preprocessor (includes are handled by the preprocessor) will see that the symbol MOO_HEADER_INCLUDE_GUARD is already defined and skip the contents of the header.

Jean-Marc Volle · Accepted Answer · 2020-09-22 07:31:26Z

1

Let's say you create a.h and a.cpp a.h

#pragma once //ensure a.h will only be included once in case multiple c++ include it
#include <vector>
void A(std::vector<int> &vect)

a.cpp

#include "a.h" // include your own declarations
void A(std::vector<int> &vect)
{
...
}

Whoever includes a.h will by a side effect also include <vector> but each module (.h) still needs to include all its own dependencies (ie if you need std::vector somewhere, you have to include it, otherwise your code might not compile any more if used in another context.

answered Sep 22, 2020 at 7:31

Jean-Marc Volle

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