Why is using std::string still needed after #include ?

Question

In order to use strings I need to include the string header, so that its implementation becomes available. But if that is so, why do I still need to add the line using std::string?

Why doesn't it already know about the string data type?

#include <string>

using std::string;

int main() {
    string s1;
}

Isn't that bad practice? I try to use `using` only for things i need. — vanmarcke, Feb 24 '17 at 07:35
@vanmarcke info to why `using namespace std` is a bad practice: http://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice?rq=1 but before, you have to read about namespace (see mpiatek's answer) — Garf365, Feb 24 '17 at 08:20

score 8 · Accepted Answer · edited Sep 29 '17 at 18:23

8

Because string is defined within namespace called std.

you can write std::string everywhere where <string> is included but you can add using std::string and don't use namespace in the scope (so std::string might be reffered to as string). You can place it for example inside the function and then it applies only to that function:

#include <string>

void foo() {
    using std::string;

    string a; //OK
}

void bar() {
    std::string b; //OK
    string c; //ERROR: identifier "string" is undefined
}

edited Sep 29 '17 at 18:23

Graham

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answered Feb 24 '17 at 07:05

mpiatek

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so the string is not defined in the string header? – vanmarcke Feb 24 '17 at 07:11
2

@vanmarcke it is defined in the string header within the std namespace. Take a look at the links at the end of my answer. – mpiatek Feb 24 '17 at 07:13

score 8 · Answer 2 · edited Feb 24 '17 at 18:47

8

using std::string; doesn't mean you can now use this type, but you can use this type without having to specify the namespace std:: before the name of the type.

The following code is correct:

#include <string>

int main()
{
    std::string s1;
    return 0;
}

edited Feb 24 '17 at 18:47

Peter Mortensen

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answered Feb 24 '17 at 07:09

Caduchon

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score 2 · Answer 3 · answered Feb 24 '17 at 07:06

2

Because the declaration of class string is in the namespace std. Thus you either need to always access it via std::string (then you don't need to have using) or do it as you did.

answered Feb 24 '17 at 07:06

dmi

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score 1 · Answer 4 · edited Feb 24 '17 at 18:46

Namespace is an additional feature of C++, which is defining the scope of a variable, function or object and avoiding the name collision. Here, the string object is defined in the std namespace.

std is the standard namespace. cout, cin, string and a lot of other things are defined in it.

The header <string> declares the various entities related to the strings library, whereas namespaces are used to group related functionality and allow use of the same names in different namespaces.

Why is using std::string still needed after #include ?

4 Answers4

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