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

Before C++11

static const int arr[] = {10, 20, 30};
vector<int> v(arr, arr + sizeof(arr) / sizeof(arr[0]));

We can also do:

vector<int> v;
v.push_back(SomeValue);
// ...

Since C++11

Using copy-list-initialization:

vector<int> v = {1, 3, 5, 7};

We can do this using direct-list-initialization well:

vector<int> v {1, 3, 5, 7}; // Notice .. no "=" sign

Since C++17

Class template argument deduction (CTAD) lets you omit the template arguments:

vector v = {1, 3, 5, 7};

Answer 2

If you can, use the modern C++[11,14,17,20,...] way:

std::vector<int> ints = {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 3

If the array is:

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

Answer 4

You can do that using boost::assign:

vector<int> values;
values += 1,2,3,4,5,6,7,8,9;

Details are here.

Answer 5

I tend to declare

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);

Answer 6

If you don't want to use Boost, but want to enjoy syntax like

std::vector<int> v;
v+=1,2,3,4,5;

just include this chunk of code

template <class T> class vector_inserter{
public:
    std::vector<T>& v;
    vector_inserter(std::vector<T>& v):v(v){}
    vector_inserter& operator,(const T& val){v.push_back(val);return *this;}
};
template <class T> vector_inserter<T> operator+=(std::vector<T>& v,const T& x){
    return vector_inserter<T>(v),x;
}

Answer 7

In C++11:

#include <vector>
using std::vector;
...
vector<int> vec1 { 10, 20, 30 };
// or
vector<int> vec2 = { 10, 20, 30 };

Using Boost list_of:

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

Using Boost assign:

#include <vector>
#include <boost/assign/std/vector.hpp>
using std::vector;
...
vector<int> vec;
vec += 10, 20, 30;

Conventional STL:

#include <vector>
using std::vector;
...
static const int arr[] = {10,20,30};
vector<int> vec (arr, arr + sizeof(arr) / sizeof(arr[0]) );

Conventional STL with generic macros:

#include <vector>
#define ARRAY_SIZE(ar) (sizeof(ar) / sizeof(ar[0])
#define ARRAY_END(ar) (ar + ARRAY_SIZE(ar))
using std::vector;
...
static const int arr[] = {10,20,30};
vector<int> vec (arr, ARRAY_END(arr));

Conventional STL with a vector initializer macro:

#include <vector>
#define INIT_FROM_ARRAY(ar) (ar, ar + sizeof(ar) / sizeof(ar[0])
using std::vector;
...
static const int arr[] = {10,20,30};
vector<int> vec INIT_FROM_ARRAY(arr);

Answer 8

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 9

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 instead.

---

// 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)

Answer 10

The below methods can be used to initialize the vector in C++.

1. int arr[] = {1, 3, 5, 6}; vector<int> v(arr, arr + sizeof(arr)/sizeof(arr[0]));

2. vector<int>v; v.push_back(1); v.push_back(2); v.push_back(3); and so on

3. vector<int>v = {1, 3, 5, 7};

The third one is allowed only in C++11 onwards.

Answer 11

For vector initialisation -

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

can be done if you have a C++11 compiler.

Else, you can have an array of the data and then use a for loop.

int array[] = {10,20,30}
for(unsigned int i=0; i<sizeof(array)/sizeof(array[0]); i++)
{
     v.push_back(array[i]);
}

Apart from these, there are various other ways described in previous answers using some code. In my opinion, these ways are easy to remember and quick to write.

Answer 12

There are various ways to hardcode a vector. I will share few ways:

1. Initializing by pushing values one by one

   // Create an empty vector
   vector<int> vect;
   
   vect.push_back(10);
   vect.push_back(20);
   vect.push_back(30);
   

2. Initializing like arrays

   vector<int> vect{ 10, 20, 30 };
   

3. Initializing from an array

   int arr[] = { 10, 20, 30 };
   int n = sizeof(arr) / sizeof(arr[0]);
   
   vector<int> vect(arr, arr + n);
   

4. Initializing from another vector

   vector<int> vect1{ 10, 20, 30 };
   
   vector<int> vect2(vect1.begin(), vect1.end());
   

Answer 13

The simplest, ergonomic way (with C++ 11 or later):

auto my_ints = {1,2,3};

Answer 14

"How do I create an STL vector and initialize it like the above? What is the best way to do so with the minimum typing effort?"

The easiest way to initialize a vector as you've initialized your built-in array is using an initializer list which was introduced in C++11.

// Initializing a vector that holds 2 elements of type int.
Initializing:
std::vector<int> ivec = {10, 20};


// The push_back function is more of a form of assignment with the exception of course
//that it doesn't obliterate the value of the object it's being called on.
Assigning
ivec.push_back(30);

ivec is 3 elements in size after Assigning (labeled statement) is executed.

Answer 15

There are a lot of good answers here, but since I independently arrived at my own before reading this, I figured I'd toss mine up here anyway...

Here's a method that I'm using for this which will work universally across compilers and platforms:

Create a struct or class as a container for your collection of objects. Define an operator overload function for <<.

class MyObject;

struct MyObjectList
{
    std::list<MyObject> objects;
    MyObjectList& operator<<( const MyObject o )
    { 
        objects.push_back( o );
        return *this; 
    }
};

You can create functions which take your struct as a parameter, e.g.:

someFunc( MyObjectList &objects );

Then, you can call that function, like this:

someFunc( MyObjectList() << MyObject(1) <<  MyObject(2) <<  MyObject(3) );

That way, you can build and pass a dynamically sized collection of objects to a function in one single clean line!

Answer 16

In case you want to have it in your own class:

#include <initializer_list>
Vector<Type>::Vector(std::initializer_list<Type> init_list) : _size(init_list.size()),
_capacity(_size),
_data(new Type[_size])
{
    int idx = 0;
    for (auto it = init_list.begin(); it != init_list.end(); ++it)
        _data[idx++] = *it;
}

Answer 17

It is pretty convenient to create a vector inline without defining variable when writing test, for example:

assert(MyFunction() == std::vector<int>{1, 3, 4}); // <- this.

Answer 18

I build my own solution using va_arg. This solution is C++98 compliant.

#include <cstdarg>
#include <iostream>
#include <vector>

template <typename T>
std::vector<T> initVector (int len, ...)
{
  std::vector<T> v;
  va_list vl;
  va_start(vl, len);
  for (int i = 0; i < len; ++i)
    v.push_back(va_arg(vl, T));
  va_end(vl);
  return v;
}

int main ()
{
  std::vector<int> v = initVector<int> (7,702,422,631,834,892,104,772);
  for (std::vector<int>::const_iterator it = v.begin() ; it != v.end(); ++it)
    std::cout << *it << std::endl;
  return 0;
}

Demo

Answer 19

// Before C++11
// I used following methods:

// 1.
int A[] = {10, 20, 30};                              // original array A

unsigned sizeOfA = sizeof(A)/sizeof(A[0]);           // calculate the number of elements

                                                     // declare vector vArrayA,
std::vector<int> vArrayA(sizeOfA);                   // make room for all
                                                     // array A integers
                                                     // and initialize them to 0 

for(unsigned i=0; i<sizeOfA; i++)
    vArrayA[i] = A[i];                               // initialize vector vArrayA


//2.
int B[] = {40, 50, 60, 70};                          // original array B

std::vector<int> vArrayB;                            // declare vector vArrayB
for (unsigned i=0; i<sizeof(B)/sizeof(B[0]); i++)
    vArrayB.push_back(B[i]);                         // initialize vArrayB

//3.
int C[] = {1, 2, 3, 4};                              // original array C

std::vector<int> vArrayC;                            // create an empty vector vArrayC
vArrayC.resize(sizeof(C)/sizeof(C[0]));              // enlarging the number of 
                                                     // contained elements
for (unsigned i=0; i<sizeof(C)/sizeof(C[0]); i++)
     vArrayC.at(i) = C[i];                           // initialize vArrayC


// A Note:
// Above methods will work well for complex arrays
// with structures as its elements.

Answer 20

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 21

In C++11:

static const int a[] = {10, 20, 30};
vector<int> vec (begin(a), end(a));

Answer 22

typedef std::vector<int> arr;

arr a {10, 20, 30};       // This would be how you initialize while defining

To compile use:

clang++ -std=c++11 -stdlib=libc++  <filename.cpp>

Answer 23

If your compiler supports C++11, you can simply do:

std::vector<int> v = {1, 2, 3, 4};

This is available in GCC as of version 4.4. Unfortunately, VC++ 2010 seems to be lagging behind in this respect.

Alternatively, the Boost.Assign library uses non-macro magic to allow the following:

#include <boost/assign/list_of.hpp>
...
std::vector<int> v = boost::assign::list_of(1)(2)(3)(4);

Or:

#include <boost/assign/std/vector.hpp>
using namespace boost::assign;
...
std::vector<int> v;
v += 1, 2, 3, 4;

But keep in mind that this has some overhead (basically, list_of constructs a std::deque under the hood) so for performance-critical code you'd be better off doing as Yacoby says.

Answer 24

In C++0x you will be able to do it in the same way that you did with an array, but not in the current standard.

With only language support you can use:

int tmp[] = { 10, 20, 30 };
std::vector<int> v( tmp, tmp+3 ); // use some utility to avoid hardcoding the size here

If you can add other libraries you could try boost::assignment:

vector<int> v = list_of(10)(20)(30);

To avoid hardcoding the size of an array:

// option 1, typesafe, not a compile time constant
template <typename T, std::size_t N>
inline std::size_t size_of_array( T (&)[N] ) {
   return N;
}
// option 2, not typesafe, compile time constant
#define ARRAY_SIZE(x) (sizeof(x) / sizeof(x[0]))

// option 3, typesafe, compile time constant
template <typename T, std::size_t N>
char (&sizeof_array( T(&)[N] ))[N];    // declared, undefined
#define ARRAY_SIZE(x) sizeof(sizeof_array(x))

Answer 25

Related, you can use the following if you want to have a vector completely ready to go in a quick statement (e.g. immediately passing to another function):

#define VECTOR(first,...) \
   ([](){ \
   static const decltype(first) arr[] = { first,__VA_ARGS__ }; \
   std::vector<decltype(first)> ret(arr, arr + sizeof(arr) / sizeof(*arr)); \
   return ret;})()

example function

template<typename T>
void test(std::vector<T>& values)
{
    for(T value : values)
        std::cout<<value<<std::endl;
}

example use

test(VECTOR(1.2f,2,3,4,5,6));

though be careful about the decltype, make sure the first value is clearly what you want.

Answer 26

The easiest way to do it is:

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

Answer 27

If your compiler supports Variadic macros (which is true for most modern compilers), then you can use the following macro to turn vector initialization into a one-liner:

#define INIT_VECTOR(type, name, ...) \
static const type name##_a[] = __VA_ARGS__; \
vector<type> name(name##_a, name##_a + sizeof(name##_a) / sizeof(*name##_a))

With this macro, you can define an initialized vector with code like this:

INIT_VECTOR(int, my_vector, {1, 2, 3, 4});

This would create a new vector of ints named my_vector with the elements 1, 2, 3, 4.

Answer 28

If you want something on the same general order as Boost::assign without creating a dependency on Boost, the following is at least vaguely similar:

template<class T>
class make_vector {
    std::vector<T> data;
public:
    make_vector(T const &val) { 
        data.push_back(val);
    }

    make_vector<T> &operator,(T const &t) {
        data.push_back(t);
        return *this;
    }

    operator std::vector<T>() { return data; }
};

template<class T> 
make_vector<T> makeVect(T const &t) { 
    return make_vector<T>(t);
}

While I wish the syntax for using it was cleaner, it's still not particularly awful:

std::vector<int> x = (makeVect(1), 2, 3, 4);

Answer 29

One method would be to use the array to initialize the vector

static const int arr[] = {16,2,77,29};
vector<int> vec (arr, arr + sizeof(arr) / sizeof(arr[0]) );