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I have a std::array<Foo, 10> and I would like to create a std::array<Bar, 10> using a function from Foo to Bar. Ordinarily I would use std::transform like so:
array<Bar, 10> bars;
transform(foos.begin(), foos.end(), bars.begin(), [](Foo foo){
return Bar(foo.m_1, foo.m_2);
});
However, Bar doesn't have a default constructor, so I can't create the bars array. I could always use vector but it would be nice to be able to use array to guarantee that that I always have exactly 10 elements. Is that possible?
772
std::array contains a built-in array, which can be initialized via an initializer list, which is what the inner set is. The outer set is for aggregate initialization.
Initialization of std::array Like arrays, we initialize an std::array by simply assigning it values at the time of declaration. For example, we will initialize an integer type std::array named 'n' of length 5 as shown below; std::array<int, 5> n = {1, 2, 3, 4, 5};
std::transform applies the given function to a range and stores the result in another range, keeping the original elements order and beginning at d_first .
The transform() function in C++ sequentially applies an operation to the elements of an array(s) and then stores the result in another output array. The transform function is used in two forms: Unary operation: The operation is applied to each element in the input range, and the result is stored in the output array.
Not with std::transform, but nothing a little template magic can't fix.
template<std::size_t N, std::size_t... Is>
std::array<Bar, N> foos_to_bars(const std::array<Foo, N>& foos,
std::index_sequence<Is...>) {
return {{ Bar(foos[Is].m_1, foos[Is].m_2)... }};
}
template<std::size_t N, std::size_t... Is>
std::array<Bar, N> foos_to_bars(const std::array<Foo, N>& foos) {
return foos_to_bars(foos, std::make_index_sequence<N>());
}
std::index_sequence and friends are C++14, but easily implementable in C++11. There are probably half a dozen implementations on SO alone.
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