Setting std :: vector in class constructor with different values for element constructor
I have a class with a non-trivial constructor:
class mystream
{
public:
mystream(size_t buffersize,size_t index) : buffersize(buffersize),index(index){}
size_t buffersize;
size_t index;
};
An instance mystream
has a unique identifier corresponding to its position in the vector of the controlling class:
class mystreammanager
{
public:
mystreammanager() : streams(8,1024, /* WHAT TO DO HERE ??? */ )
{
}
std::vector<mystream> streams;
};
How can I construct a vector and initialize its elements with ascending value for the index?
Concise, clear and easy to debug the way to do this is to defer vector construction to a static class function:
class mystreammanager
{
public:
mystreammanager() : streams{ generate_streams(1024, 8) }
{
}
private:
static std::vector<mystream> generate_streams(size_t buffersize, size_t qty)
{
std::vector<mystream> result;
result.reserve(qty);
for(size_t i = 0 ; i < qty ; ++qty) {
result.emplace_back(buffersize, i);
}
return result;
}
std::vector<mystream> streams;
};
This is optimally effective because:
- RVO calls vector creation in place
- excluding list initialization means no redundant copies.
Just use a loop:
mystreammanager() {
streams.reserve(8);
for (int i = 0; i < 8; ++i) {
streams.emplace_back(1024, i);
}
}
Compile time version :)
Required c++14
, but can certainly be adapted forc++11
#include <cstddef>
#include <vector>
#include <utility>
class mystream
{
public:
mystream(size_t buffersize,size_t index) : buffersize(buffersize),index(index){}
size_t buffersize;
size_t index;
};
template<size_t... Indexes>
std::initializer_list<mystream> mystream_maker_impl(std::index_sequence<Indexes...>)
{
return {{1024, Indexes}...};
}
template<size_t N>
std::initializer_list<mystream> mystream_maker()
{
return mystream_maker_impl(std::make_index_sequence<N>());
}
class mystreammanager
{
public:
mystreammanager() : streams(mystream_maker<8>())
{
}
std::vector<mystream> streams;
};
You can do:
class mystreammanager
{
public:
mystreammanager() : streams{{1024, 0}, {1024, 1}, {1024, 2}, {1024, 3},
{1024, 4}, {1024, 5}, {1024, 6}, {1024, 7}}
{
}
std::vector<mystream> streams;
};
But doing the loop seems safer / easier.
I used the answer from @RichardHodges as I was unhappy with my first choice. I came up with this pattern:
template<class T,class ...Args> std::vector<T> generate_with_index(size_t qty,Args ...args)
{
std::vector<T> result;
result.reserve(qty);
for(size_t i = 0 ; i < qty ; ++qty)
result.emplace_back(i, args...);
return result;
}
This helps me avoid redundancy. From a theoretical point of view, I like @ Drax's solution better, as it works most efficiently at compile time.