Initialize array or vector over constructor's initializing list

Question

How can I initialize a (string) array or vector with a constructor's initializing list in C++?

Consider please this example where I want to initialize a string array with arguments given to the constructor:

#include <string>
#include <vector>

class Myclass{
           private:
           std::string commands[2];
           // std::vector<std::string> commands(2); respectively 

           public:
           MyClass( std::string command1, std::string command2) : commands( ??? )
           {/* */}
}

int main(){
          MyClass myclass("foo", "bar");
          return 0;
}

Besides that, which of the two types (array vs vector) is recommended for saving two strings while creating an object, and why?

@chris: As in Myclass m = {"hello", "world"}; or as in Myclass(const std::string& c1, const std::string& c2) : commands{c1, c2}{}? — thokra
– thokra, Commented Oct 24, 2013 at 13:38
@thokra, One of those won't initialize the members, while the other is displayed nicely in Vaughn's answer. — Qaz
– Qaz, Commented Oct 24, 2013 at 13:41

Vaughn Cato · Accepted Answer · 2013-10-24 13:47:33Z

12

With C++11, you do it like this:

class MyClass{
           private:
           std::string commands[2];
           //std::vector<std::string> commands;

           public:
           MyClass( std::string command1, std::string command2)
             : commands{command1,command2}
           {/* */}
};

For pre-C++11 compilers, you will need to initialize the array or vector in the body of the constructor:

class MyClass{
           private:
           std::string commands[2];

           public:
           MyClass( std::string command1, std::string command2)
           {
               commands[0] = command1;
               commands[1] = command2;
           }
};

or

class MyClass{
           private:
           std::vector<std::string> commands;

           public:
           MyClass( std::string command1, std::string command2)
           {
               commands.reserve(2);
               commands.push_back(command1);
               commands.push_back(command2);
           }
};

edited Oct 24, 2013 at 13:47

answered Oct 24, 2013 at 13:37

Vaughn Cato

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4 Comments

fotinsky Over a year ago

Thanks. And for older compilers?

Vaughn Cato Over a year ago

@fotinsky: I've extended my answer to cover pre-C++11 compilers.

Leo Fang Over a year ago

@VaughnCato, what if I use C++11 compilers and define a private member std::vector<std::string> commands; as you did in the pre-C++ example? Is there a faster way to initialize commands?

Vaughn Cato Over a year ago

@leofang yes. It's exactly the same either way.

Ivaylo Strandjev · Accepted Answer · 2013-10-24 13:31:12Z

1

In the initializer list you may call any constructor of the class of the member you want to initialize. Take a look at std::string and std::vector documentation and choose the constructor that works for you.

For storing two objects I would recommend using std::pair. However if you expect that the number may grow std::vector is the best option.

answered Oct 24, 2013 at 13:31

Ivaylo Strandjev

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Comments

titus · Accepted Answer · 2013-10-24 13:32:47Z

1

You could use

#include <utility>

...
std::pair<string, string> commands;
commands=std::make_pair("string1","string2");

...
//to access them use
std::cout<<commands.first<<" "<<commands.second;

answered Oct 24, 2013 at 13:32

titus

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Comments

Caesar · Accepted Answer · 2013-10-24 13:49:03Z

1

class Myclass{
private:
    Vector<string> *commands;
    // std::vector<std::string> commands(2); respectively 

    public:
    MyClass( std::string command1, std::string command2)
    {
        commands.append(command1);  //Add your values here
    }
}

edited Oct 24, 2013 at 13:49

Caesar

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answered Oct 24, 2013 at 13:35

anbu selvan

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