creating a vector with user defined class and int In instantiation of ‘struct std::_Vector_base....no type named ‘value_type’ in ‘class MyClass’

user_4808760 0
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I am creating a simple class

#include <iostream>
#include <vector>
#include <algorithm>    

using namespace std;

class MyClass {
    int p;

public:
    MyClass( int q ) { p = q; }
};

but when I try to create this vector

 vector<int, MyClass> vec1(1);

my code breaks and throws this exception

vector<int,MyClass> vec1(1);

In instantiation of ‘struct std::_Vector_base<int, MyClass>’:
/usr/include/c++/9/bits/stl_vector.h:386:11:   required from ‘class std::vector<int, MyClass>’
vector_class.cpp:19:26:   required from here
/usr/include/c++/9/bits/stl_vector.h:84:21: error: no type named ‘value_type’ in ‘class MyClass’
   84 |  rebind<_Tp>::other _Tp_alloc_type;

Can I resolve this means can I use int type with user defined types in vector like this vector<int,MyClass>

stl c++
2 Answers

vector has two template parameters.

  • The first is the type of elements to be stored into the vector.
  • The second is an allocator, a helper class that allocates and frees memory used by the vector.

You are trying to create a vector that uses MyClass as the allocator. However, MyClass does not provide the calls that the vector expects, so it fails to compile.

What are you trying to accomplish?

If you want a vector containing objects of MyClass, then you should declare a vector<MyClass>. If you want a vector that contains pairs of objects, an int and a MyClass then you can use vector<pair<int, MyClass> (as the other commenters have mentioned).

But it is not clear from your question what you really want.

user_9924900

how to print p if I am using pair in vector as u shown

You can iterate through the pair elements of the vector and print the data member p as shown below:

class MyClass {
    public:
    int p;

public:
    MyClass( int q ):p(q)
    {
        
    }
};

int main()
{
    std::vector<std::pair<int, MyClass>> vec1{{1, MyClass(44)}, {2,MyClass(45)}, {3,MyClass(46)}};
    
    //iterate through the vector and print data member p
    for(const auto&myPair: vec1)
    {
//-----------------vvvvvvvvvvvvvv-------------->print the data member p 
        std::cout<<myPair.second.p<<std::endl;
    }
}

Working demo

user_120025700
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