inserting a class object into a map??

[dorito200]

Hey yall, so we have an assignment dealing with maps, and I think I understand them, but I cannot get this one to work. I have a class (Apps) that contains a string for the name, a string for the type, and a float for the cost, and I want the name string to be the Key value for the map. I think I initialized the map correctly, but on line 2 I am getting the error "missing template arguments before '(' token". I understand that means I'm missing some a character somewhere, but in all the examples I'm seeing I have exactly what they have. I'd also like to insert already constructed objects, but I don't know if that is possible considering the emplace function. If anyone has any ideas or thoughts I'd really appreciate them!

  std::map<string, Apps> Appmap;
  Appmap.insert(std::pair(1, Apps(AppName[0], AppPrice[0], AppType[0])));

[Ganado]

For the syntactical issue, it's saying you need std::pair<TypeA, TypeB>(...) and not just std::pair(...). I think this can be automatically determined by the compiler in either C++17 or C++20, but don't quote me on that.

But notice your map is a mapping of string to 'Apps', but the first element of your pair is an int. If we change the map to <int, Apps>, then things work better.

// Example program
#include <iostream>
#include <string>
#include <map>
#include <utility>

using std::string;

struct Apps {
    Apps(string name, int price, string type)
    : name(name), price(price), type(type) { }
    
    bool operator<(const Apps& other)
    {
        return name < other.name;   
    }
    
    string name;
    int price;
    string type;  
};

int main()
{
    string AppName[] = { "TestName", "Unused" };
    int AppPrice[] = { 42, 0 };
    string AppType[] = { "TestType", "Unused" };
    
    std::map<int, Apps> Appmap;
    Appmap.insert(std::pair<int, Apps>(1, Apps(AppName[0], AppPrice[0], AppType[0]))); 
}

I'd also like to insert already constructed objects

There's nothing really special needed to do this. You'd just create some named variable before the insert call.

    Apps my_apps(AppName[0], AppPrice[0], AppType[0]);
    Appmap.insert(std::pair<int, Apps>(1, my_apps)); 

[keskiverto]

If you want to use name as key, then do so:

std::map<string, Apps> Appmap;
Appmap.insert( std::make_pair( AppName[0], Apps(AppName[0], AppPrice[0], AppType[0]) ) );
Appmap[ AppName[1] ] = Apps(AppName[1], AppPrice[1], AppType[1]); // less efficient for add 

The std::make_pair is a function template and picks the types from arguments unlike class template std::pair.

[Peter87]

Ganado, is right. You don't need to specify the template arguments to std::pair thanks to class template argument deduction (CTAD) that was introduced in C++17. The only problem with your code (using a modern compiler) is that you pass an int and not a string as the first argument when constructing the std::pair.

std::map<std::string, Apps> Appmap;
Appmap.insert(std::pair(AppName[0], Apps(AppName[0], AppPrice[0], AppType[0])));

You can even leave out std::pair and just put the arguments inside { }. The compiler knows that the insert function expects a std::pair<std::string, Apps> and will automatically deduce that is what you want to construct.

std::map<std::string, Apps> Appmap;
Appmap.insert({AppName[0], Apps(AppName[0], AppPrice[0], AppType[0])});

[dorito200]

Thank you all for the responses, Ganado was very helpful but Peter87 had it really clean too, thanks to the both of you for the help!