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 sim

Answer 1

If the array is:

int arr[] = {1, 2, 3};
int len = (sizeof(arr)/sizeof(arr[0])); // finding length of array
vector < int > v;
std:: v.assign(arr, arr+len); // assigning elements from array to vector 

Answer 2

B. Stroustrup describes a nice way to chain operations in 16.2.10 Selfreference on page 464 in the C++11 edition of the Prog. Lang. where a function returns a reference, here modified to a vector. This way you can chain like v.pb(1).pb(2).pb(3); but may be too much work for such small gains.

#include <iostream>
#include <vector>

template<typename T>
class chain
{
private:
    std::vector<T> _v;
public:
    chain& pb(T a) {
        _v.push_back(a);
        return *this;
    };
    std::vector<T> get() { return _v; };
};

using namespace std;

int main(int argc, char const *argv[])
{
    chain<int> v{};

    v.pb(1).pb(2).pb(3);

    for (auto& i : v.get()) {
        cout << i << endl;
    }

    return 0;
}

1
2
3

Answer 3

If you can, use the modern C++[11,14,17,...] way:

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

The old way of looping over a variable-length array or using sizeof() is truly terrible on the eyes and completely unnecessary in terms of mental overhead. Yuck.

Answer 4

Just thought I'd toss in my $0.02. I tend to declare this:

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

in a utility header somewhere and then all that's required is:

const double values[] = { 2.0, 1.0, 42.0, -7 };
std::vector<double> array = makeVector(values);

But I can't wait for C++0x. I'm stuck because my code must also compile in Visual Studio. Boo.

Answer 5

Starting with:

int a[] = {10, 20, 30}; //i'm assuming a is just a placeholder

If you don't have a C++11 compiler and you don't want to use boost:

const int a[] = {10, 20, 30};
const std::vector<int> ints(a,a+sizeof(a)/sizeof(int)); //make it const if you can

If you don't have a C++11 compiler and can use boost:

#include <boost/assign.hpp>
const std::vector<int> ints = boost::assign::list_of(10)(20)(30);

If you do have a C++11 compiler:

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

Answer 6

A more recent duplicate question has this answer by Viktor Sehr. For me, it is compact, visually appealing (looks like you are 'shoving' the values in), doesn't require c++11 or a third party module, and avoids using an extra (written) variable. Below is how I am using it with a few changes. I may switch to extending the function of vector and/or va_arg in the future intead.

---

// Based on answer by "Viktor Sehr" on Stack Overflow
// https://stackoverflow.com/a/8907356
//
template <typename T>
class mkvec {
public:
    typedef mkvec<T> my_type;
    my_type& operator<< (const T& val) {
        data_.push_back(val);
        return *this;
    }
    my_type& operator<< (const std::vector<T>& inVector) {
        this->data_.reserve(this->data_.size() + inVector.size());
        this->data_.insert(this->data_.end(), inVector.begin(), inVector.end());
        return *this;
    }
    operator std::vector<T>() const {
        return data_;
    }
private:
    std::vector<T> data_;
};

std::vector<int32_t>    vec1;
std::vector<int32_t>    vec2;

vec1 = mkvec<int32_t>() << 5 << 8 << 19 << 79;  
// vec1 = (5,8,19,79)
vec2 = mkvec<int32_t>() << 1 << 2 << 3 << vec1 << 10 << 11 << 12;  
// vec2 = (1,2,3,5,8,19,79,10,11,12)