I want to initialize a vector like we do in case of an array.
Example
int vv[2] = {12, 43}; But when I do it like this,
vector<int> v(2) = {34, 23}; OR
vector<int> v(2); v = {0, 9}; it gives an error:
expected primary-expression before ‘{’ token
AND
error: expected ‘,’ or ‘;’ before ‘=’ token
respectively.
With the new C++ standard (may need special flags to be enabled on your compiler) you can simply do:
std::vector<int> v { 34,23 }; // or // std::vector<int> v = { 34,23 }; Or even:
std::vector<int> v(2); v = { 34,23 }; On compilers that don't support this feature (initializer lists) yet you can emulate this with an array:
int vv[2] = { 12,43 }; std::vector<int> v(&vv[0], &vv[0]+2); Or, for the case of assignment to an existing vector:
int vv[2] = { 12,43 }; v.assign(&vv[0], &vv[0]+2); Like James Kanze suggested, it's more robust to have functions that give you the beginning and end of an array:
template <typename T, size_t N> T* begin(T(&arr)[N]) { return &arr[0]; } template <typename T, size_t N> T* end(T(&arr)[N]) { return &arr[0]+N; } And then you can do this without having to repeat the size all over:
int vv[] = { 12,43 }; std::vector<int> v(begin(vv), end(vv)); 
std::vector<int> v(begin(w), end(w);. The begin and end are standard in C++11 (but then you don't need them), but should be in your tool kit otherwise.
std::vector<int> v(&vv[0], &vv[0]+2); ? What I see is, you're constructing a vector with room for &vv[0] (which will be a memory address) values, and filling each space in the vector with &vv[0]+2... That would be using constructor 2 on this page: en.cppreference.com/w/cpp/container/vector/vector without supplying the third argument, which defaults to Allocator(). I know I'm missing something.
std::vector<int> v(&vv[0], &vv[0]+2) is invoking the 4th constructor on that page, actually. The constructor can take the first and last element in a range and create a vector with everything in between. The tipoff is the & will result in memory addresses.
You can also do like this:
template <typename T> class make_vector { public: typedef make_vector<T> my_type; my_type& operator<< (const T& val) { data_.push_back(val); return *this; } operator std::vector<T>() const { return data_; } private: std::vector<T> data_; }; And use it like this:
std::vector<int> v = make_vector<int>() << 1 << 2 << 3; 
g++ -std=c++11 your_file.cc. Then you can use initializer list constructor of the thread (the last item in this reference)