What is the easiest way to initialize a std::vector with hardcoded elements?

Question

I can create an array and initialize it like this: int a[] = {10, 20, 30}; How do I create a `std::vector` and initialize it similarly elegant? The best way I know is: std::vector<int> ints; ints.push_back(10); ints.push_back(20); ints.push_back(30); Is there a better way?

Answer 1

OK, there are two questions here:

all of the C++ standard library collection classes (std::vector<T>, std::list<T>, ...) have that behavior built-in since C++11.

Be sure that you have that compiler setting turned on, like such

e.g., if your file is called vect.cpp, you would compile it from the command line with unix as:

g++ vect.cpp -std=c++11 -o vect

and then run it as:

./vect

Here's the code:

#include <iostream>

#include <vector>

int main(int argc, const char* argv[]) {

std::vector<int> v = { 10, 20, 30 };

for (int el : v) {

std::cout << el << " ";

}

std::cout << "\n";

return 0;

}

Second question: What if you wanted to create a CUSTOM CLASS, and have that behavior built in? Then you would do the following:

#include <iostream>

#include <vector>

#include <initializer_list>

class collection {

public:

// use default ctor (which calls default ctor for std::vector<T>)

collection() = default;

// call the default constructor BEFORE using the std::initializer_list<T>

collection(const std::initializer_list<int>& li)

: collection() {

for (int el : li) {

push_back(el);

}

}

void push_back(int val) { v.push_back(val); } // std::vector<int> does work

bool empty() const { return v.empty(); } // ditto

size_t size() const { return v.size(); } // ditto

friend std::ostream& operator<<(std::ostream& os, const collection& c) {

if (c.empty()) { return os << "collection is empty\n"; }

for (int el : c.v) {

os << el << " ";

}

return os << "\n";

}

private:

std::vector<int> v;

};

int main(int argc, const char* argv[]) {

collection c = { 4, 17, 28, 33 }; // use init list for CUSTOM collection class

std::cout << "c is: " << c << "\n";

return 0;

}