使用模板参数初始化STL类

Question

I am trying to do declare a stl map with template parameters like so: ( assume T as typename like so : template <class T> )

map<T, T> m; ( in .h file )

It compiles fine. now in my cpp file, when I want to insert into the map, i am not able to. The only methods i get on intellisense are "at" and "swap" methods.

Any ideas? Anyone please?

Thanks in advance.

here is sample code:

#pragma once

#include <iostream>
#include <map>

using namespace std;

template <class T> 

class MySample  
{  
map<T, T> myMap;
//other details omitted

public:

//constructor 
MySample(T t)
{
    //here I am not able to use any map methods. 
    //for example i want to insert some elements into the map
    //but the only methods I can see with Visual Studio intellisense
    //are the "at" and "swap" and two other operators
    //Why???
    myMap.  
}

//destructor
~MySample(void)
{

}
//other details omitted
};

Answer 1

The usual ways to insert key-value pairs into a std::map are the index-operator syntax as well as the insert function. I'll assume std::string for keys and int for values for the sake of the example:

#include <map>
#include <string>

std::map<std::string,int> m;
m["hello"] = 4;  // insert a pair ("hello",4)
m.insert(std::make_pair("hello",4)); // alternative way of doing the same

If you can use C++11, you may use the new uniform initialization syntax instead of the make_pair call:

m.insert({"hello",4});

And, as said in the comments, there is

m.emplace("hello",4);

in C++11, which constructs the new key-value pair in-place, rather than constructing it outside the map and copying it in.

---

I should add that since your question is actually about initialization , rather than inserting fresh elements, and given that indeed you do this in the constructor of MyClass , what you should really do (in C++11) is this:

MySample(T t)
 : myMap { { t,val(t) } }
{}

(Here I assume there is some function val which generates the value to store for t in the map.)