use of vector<int>::value_type?

[w3rd]

Hi,

I wonder what is the use of value_type?
If I type
vector<int>::value_type i;
i is an int right? But I had to specifiy the int in vector<int> anyway. So what is the use of that?
I could have done
int i;
instead. I guess I am missing something.
Thank you!

PS: ok I guess this comes in to play when I get to templates.

[laserlight]

ok I guess this comes in to play when I get to templates.

Yes, I believe that is the idea, e.g., if the container type is T, you could get the value type as typename T::value_type.

[w3rd]

what do you mean by container type T? Like vector or list?
vector::value_type wouldnt make much sense, would it?
Well I will be patient still some hundred pages until templates...

[JohnW@Wessex]

value_type is used in templated functions and classes where it is the only way to determine the type used by a template parameter.

template <typename T>
T::value_type Function(const T &t)
{
    T::value_type i;
    ...

    return i;
}

[laserlight]

what do you mean by container type T? Like vector or list?
vector::value_type wouldnt make much sense, would it?

Say, std::vector<int>, std::list<double>, std::vector<long, some_allocator_type>, etc.

By the way, JohnW@Wessex's example is missing the typename keyword before typename T::value_type.

[Speedo]

value_type and similar typedefs are useful for writing generic code that can work with any STL container. Eg:

#include <vector>
#include <list>
#include <iostream>

template<typename Container>
class Foo
{
  Container m_con;

public:
  Foo(const Container& c) : m_con(c) { }

  typename Container::value_type
    GetFirst( ) { return *(m_con.begin( )); }
};

int main( )
{
  std::vector<int> a;
  a.push_back(20);

  std::list<float> b;
  b.push_back(1.234f);

  Foo<std::vector<int> > c(a);
  Foo<std::list<float> > d(b);

  std::cout << c.GetFirst( ) << std::endl;
  std::cout << d.GetFirst( ) << std::endl;
  
  return 0;
}

[w3rd]

So JohnW: your Funtion() would be used like

std::vector<int> ivec;
std::vector<int>::value_type result = Function(ivec)

right? But it would work for different sequential containers with different value_types.

ok thank you both.
I am seeing it like through a mist now
Will get there.


OK one last question. Is there a difference between:
std::vector<int>::value_type search_value;
and
int search_value; ?

[Speedo]

std::vector<int>::value_type search_value;
and
int search_value; ?

No, they're both ints. Outside of templates the main reason to use value_type would be with typedefs. If you use something like

typedef std::vector<int> DataVector;

DataVector vec;
DataVector::value_type value;
cin >> value;
vec.push_back(value);

Then if you needed to change the type of data later on, say from int to double, you would only need to change the typedef.

[w3rd]

Hey Speedo, thanks! I thought that it could be used to do that but I could not get a hold of how exactly. I tried

vec::value_type

which didnt work.