initialise empty map in class constructor

[blendero777]

hey coders, sorry as i am fairly new to c++ but i have a private map in a class, I am trying to initialise the map to be empty when the constructor of the class is called e.g.

//Student.h
Class Student:

public:

private:
map <string, float> marks;


//Student.cpp
Student::Student()
{
//my attempt which throws an error
map<string, float> Student::marks;
}

I have searched but cant find a solution, am I meant to initialise it using initialisation lists within the constructor?

[Ganado]

It will be default initialized to be empty, you don't have to do anything special.

If you wanted to explicitly default initialize it, you would something like:

Student::Student()
: marks{}
{ }

Otherwise, you don't even need to define a constructor.

Edit: Note that in your code as it looks right now, your default constructor is private. Not sure if that's intentional. Edit 2: Oh wait, you don't actually show the declaration, nevermind.

// Example program
#include <iostream>
#include <string>
#include <map>
using std::map;
using std::string;

class Student {   
public: 
    map <string, float> marks;
};

int main()
{
    Student sally;
    sally.marks["History"] = 3.2f;
}

[keskiverto]

The std::map has constructors: http://www.cplusplus.com/reference/map/map/map/

The default constructor creates an empty map.

#include <map>
int main() {
  std::map<char,int> example; // Calls default constructor
  // assert: example.size() == 0
}

If you don't define any custom constructors for your class, then compiler defines a default constructor for it implicitly.
That constructor "calls the default constructors of the bases and of the non-static members of this class".
https://en.cppreference.com/w/cpp/language/default_constructor

#include <map>

class Student {
  std::map<char,int> dict;
};

int main() {
  Student poet; // Calls default constructor, which calls default constructor of dict
  // assert: poet.dict.size() == 0
}

If you define a constructor:

Student::Student
{
}

It will call default constructor of the member dict.
https://en.cppreference.com/w/cpp/language/initializer_list

You can do that explicitly too and preferably use the initializer list:

Student::Student
 : dict()
{
}

[blendero777]

ahh okay it is called by default, I want to explicitly initialise it for the sake of my assignment, the student class is a derived class, whats the correct syntax for this

//Student.cpp
Student::Student(const string &name, int regNo)
    :Person(name) 
    // I tried ( :Person(name), : dict() ) 
    //but I got a compiler error


{


}

thanks for the assistance, greatly appreciated

[mbozzi]

Student::Student(const string &name, int regNo)
    : person(name)
    , dict()
{}

https://en.cppreference.com/w/cpp/language/initializer_list

[keskiverto]

class P {
  string nam;
public:
  P( const string & name ) : nam( name ) {}
};

class S : public P {
  int num;
public:
  P( const string & sign )
  // comma-separated list:
  : P( sign ), // base's ctor
    num( 42 ) // member(s)
  {}
};

[Bra599]

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