Question

I’m fairly new to C++, but my understanding is that a #include statement will essentially just dump the contents of the #included file into the location of that statement. This means that if I have a number of ‘#include’ and ‘using’ statements in my header file, my implementation file can just #include the header file, and the compiler won’t mind if I don’t repeat the other statements.

What about people though?

My main concern is that if I don’t repeat the ‘#include’, ‘using’, and also ‘typedef’ (now that I think of it) statements, it takes that information away from the file in which it’s used, which could lead to confusion.

I am just working on small projects at the moment where it won’t really cause any issues, but I can imagine that in larger projects with more people working on them it could become a significant issue.

An example follows:

UPDATE: my function prototypes for ‘Unit’ have string, ostream and StringSet among their return types and parameters – I am not including anything in my header file that is used only in the implementation file.

//Unit.h

#include <string>
#include <ostream>
#include "StringSet.h"

using std::string;
using std::ostream;

class Unit {

public:
    //public members with string, ostream and StringSet
    //in their return values/parameter lists
private:
    //private members
    //unrelated side-question: should private members
    //even be included in the header file?
} ;


//Unit.cpp

#include "Unit.h"

//The following are all redundant from a compiler perspective:
#include <string>
#include <ostream>
#include "StringSet.h"

using std::string;
using std::ostream;

//implementation goes here

Answer 1

A using-directive (using namespace std;) should not reside in a header unless it is contained within a function. It is bad practice. It is unlikely that every user of your header wants unqualified lookup for everything in a given namespace; the inclusion of unrelated headers can lead to unexpected ambiguity and compilation failures. Personally, I avoid the using-directive inside of functions for the same reasoning, but this is generally considered less harmful.

A type alias (either through typedef std::string string; or using string = std::string;) should be used carefully. Type definitions have meaning, so you should never redeclare it. For example, this is an error:

typedef int   myint;
typedef float myint;

because of conflicting types.

A using-declaration (using std::string; or using std::memcpy;) makes a symbol accessible for unqualified name lookup. It is extremely useful when getting for argument-dependent lookup correct, which usually doesn’t matter unless you’re writing a library. The advice is different depending on if you are bringing in a type or a function. Think of using-declarations with types in the same manner as a type alias: It does not make sense to have multiple definitions under the same name. With functions, all you are really doing is extending overload resolution to include a few more things (although it is usually not necessary).

// Finding multiple operator<< functions makes sense
using std::operator<<;
using mylib::operator<<;

// Finding multiple string classes does not make sense
using std::string;
using mylib::string;

For repeating #include, you should consider if you actually need to include the file in the header in the first place. Perhaps a forward declaration fits your needs.