Initialising a vector of std::string with an array

Question

I wish to initialise a vector using an array of std::strings.

I have the following solution, but wondered if there's a more elegant way of doing this?

std::string str[] = { "one", "two", "three", "four" };
vector< std::string > vec;
vec = vector< std::string >( str, str + ( sizeof ( str ) /  sizeof ( std::string ) ) );

I could, of course, make this more readable by defining the size as follows:

int size =  ( sizeof ( str ) /  sizeof ( std::string ) );

and replacing the vector initialisation with:

vec = vector< std::string >( str, str + size );

But this still feels a little "inelegant".

See stackoverflow.com/questions/231491/…

Alan Stokes
– Alan Stokes

2012-01-26 14:14:12 +00:00
Commented Jan 26, 2012 at 14:14 — Alan Stokes
– Alan Stokes, Commented Jan 26, 2012 at 14:14

Mark B · Accepted Answer · 2012-01-26 14:12:57Z

4

Well the intermediate step isn't needed:

std::string str[] = { "one", "two", "three", "four" };
vector< std::string > vec( str, str + ( sizeof ( str ) /  sizeof ( std::string ) ) );

In C++11 you'd be able to put the brace initialization in the constructor using the initializer list constructor.

answered Jan 26, 2012 at 14:12

Mark B

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1 Comment

Kris Dunning Over a year ago

Thanks, I wasn't certain whether I'd completely missed any other potential methods of initialising things.

Mike Seymour · Accepted Answer · 2012-01-26 14:23:30Z

3

In C++11, we have std::begin and std::end, which work for both STL-style containers and built-in arrays:

#include <iterator>

std::vector<std::string> vec(std::begin(str), std::end(str));

although, as mentioned in the comments, you usually won't need the intermediate array at all:

std::vector<std::string> vec {"one", "two", "three", "four"};

In C++03, you could use a template to deduce the size of the array, either to implement your own begin and end, or to initialise the array directly:

template <typename T, size_t N>
std::vector<T> make_vector(T &(array)[N]) {
    return std::vector<T>(array, array+N);
}

std::vector<std::string> vec = make_vector(str);

edited Jan 26, 2012 at 14:23

answered Jan 26, 2012 at 14:17

Mike Seymour

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4 Comments

Konrad Rudolph Over a year ago

In C++11 we have proper initialisers, no need for the first of your methods.

Mike Seymour Over a year ago

@KonradRudolph: good point (although the question does specifically ask about initialising from an array, so I'll leave that method in the answer).

James Kanze Over a year ago

Long before C++11, we had begin and end in our personal tool kit. And they're mostly relevant to pre-C++11.

Kris Dunning Over a year ago

Thanks for this. It doesn't quite fit into the code I'm currently working on, but it is good to know. I should also have stated that I am not using C++11, but again, thank you for your help!

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