What requires me to declare “using namespace std;”

Question

This question may be a duplicate, but I can't find a good answer. Short and simple, what requires me to declare

using namespace std;

in C++ programs?

Answer 1

First of all, this is not required in C - C does not have namespaces. In C++, anything in the std namespace which includes most of the standard library. If you don't do this you have to access the members of the namespace explicitly like so:

std::cout << "I am accessing stdout" << std::endl;

Answer 2

Technically, you might be required to use using (for whole namespaces or individual names) to be able to use Argument Dependent Lookup.

Consider the two following functions that use swap().

#include <iostream>
#include <algorithm>

namespace zzz
{
    struct X {};


void swap(zzz::X&, zzz::X&) 
{
    std::cout << "Swapping X\n";
}
}

template <class T>
void dumb_swap(T& a, T& b)
{
    std::cout << "dumb_swap\n";
    std::swap(a, b);
}

template <class T>
void smart_swap(T& a, T& b)
{
    std::cout << "smart_swap\n";
    using std::swap;
    swap(a, b);
}

int main()
{
    zzz::X a, b;
    dumb_swap(a, b);
    smart_swap(a, b);

    int i, j;
    dumb_swap(i, j);
    smart_swap(i, j);
}

dumb_swap always calls std::swap - even though we'd rather prefer using zzz::swap for zzz::X objects.

smart_swap makes std::swap visible as a fall-back choice (e.g when called with ints), but since it doesn't fully qualify the name, zzz::swap will be used through ADL for zzz::X.

---

Subjectively, what forces me to use using namespace std; is writing code that uses all kinds of standard function objects, etc.

//copy numbers larger than 1 from stdin to stdout
remove_copy_if(
    std::istream_iterator<int>(std::cin), std::istream_iterator<int>(),
    std::ostream_iterator<int>(std::cout, "\n"),
    std::bind2nd(std::less_equal<int>(), 0)
);

IMO, in code like this std:: just makes for line noise.

I wouldn't find using namespace std; a heinous crime in such cases, if it is used in the implementation file (but it can be even restricted to function scope, as in the swap example).

Definitely don't put the using statement in the header files. The reason is that this pollutes the namespace for other headers, which might be included after the offending one, potentially leading to errors in other headers which might not be under your control. (It also adds the surprise factor: people including the file might not be expecting all kinds of names to be visible.)

Answer 3

All the files in the C++ standard library declare all of its entities within the std namespace.
e.g: To use cin,cout defined in iostream

Alternatives:

using std::cout;
using std::endl;
cout << "Hello" << endl;

std::cout << "Hello" << std::endl;

Answer 4

You should definitely not say:

using namespace std;

in your C++ headers, because that beats the whole point of using namespaces (doing that would constitute "namespace pollution"). Some useful resources on this topic are the following:

1) stackoverflow thread on Standard convention for using “std”

2) an article by Herb Sutter on Migrating to Namespaces

3) FAQ 27.5 from Marshall Cline's C++ Faq lite.

Answer 5

Firstly, the using directive is never required in C since C does not support namespaces at all.

The using directive is never actually required in C++ since any of the items found in the namespace can be accessed directly by prefixing them with std:: instead. So, for example:

using namespace std;
string myString;

is equivalent to:

std::string myString;

Whether or not you choose to use it is a matter of preference, but exposing the entire std namespace to save a few keystrokes is generally considered bad form. An alternative method which only exposes particular items in the namespace is as follows:

using std::string;
string myString;

This allows you to expose only the items in the std namespace that you particularly need, without the risk of unintentionally exposing something you didn't intend to.

Answer 6

Namespaces are a way of wrapping code to avoid confusion and names from conflicting. For example:

File common1.h:

namespace intutils
{
    int addNumbers(int a, int b)
    {
        return a + b;
    }
}

Usage file:

#include "common1.h"    
int main()
{
    int five = 0;
    five = addNumbers(2, 3); // Will fail to compile since the function is in a different namespace.
    five = intutils::addNumbers(2, 3); // Will compile since you have made explicit which namespace the function is contained within.

    using namespace intutils;
    five = addNumbers(2, 3); // Will compile because the previous line tells the compiler that if in doubt it should check the "intutils" namespace.
}

So, when you write using namespace std all you are doing is telling the compiler that if in doubt it should look in the std namespace for functions, etc., which it can't find definitions for. This is commonly used in example (and production) code simply because it makes typing common functions, etc. like cout is quicker than having to fully qualify each one as std::cout.