I know that string type exists in namespace std. I also know there is the <string> library which needs to be included in order to use the string type. My question is: why do i need both (to include the library and use the std namespace) to define a string? Why cant i just use it by including the library? More generally, what does the namespace std contains that is missing from the library?
2 Answers
Namespaces and headers are two separate concepts doing entirely different things:
- The header
<string>declares the various entities related to the strings library like the class templatestd::basic_string, sometypedefs likestd::stringandstd::wstring, and some functions. - Namespaces are used to group related functionality and allow use of the same names in different namespaces. For example, a "vector" is essentially an array for some (this is the view taken by the standard C++ library defining this name in namespace
std) while in other contexts it is a mathematical entity.
When you included <string> you got all entities related to the strings library declared inside namespace std. Some people object to the idea to use std::string because it is so much easier to use string instead. To aide this practice a using directive, i.e., a statement like using namespace std; can be used to look in namespace std for entities when looking up names. That is, the using directive merely makes names available without qualification.
Comments
Look at it this way: std::string is the complete name of the type, much like "M. Kostas" is your complete name.
#include <string> gives you std::string. You can call it string only if you have first established the correct context, e.g. by using namespace std or by using std::string, or if you yourself happen to be library code within namespace std { /* ... */ }.
Again, consider the analogy with family names. I can only refer to you by "M." if I have first established that I am talking about the "Kostas" family (= using namespace std), or if I previously said that by "M." I actually mean "M. Kostas" (= using std::string) , or if I am myself a member of the "Kostas" family (= namespace std { /* ... */ }).
Note that you should (in the opinion of many programmers, at least, mine included) rarely use using namespace std or using std::string. It's often better to just use complete names everywhere. At global scope in header files, using shortcuts is even no longer a matter of style but can lead to real technical problems.
using namespace std;. You can qualify the namespace directly with the type. Just writestd::string s;.stringis not a template class,basic_stringis.