Boost transformed conversion to vector error - c++

Question

I am new to using boost adapters, I am using the following code for conversion of a vector one class to transformed version.
The return type of boost::transformed is not as per expectation. Can someone please illuminate as to what I am missing here:
class blabla
{
public:
int x;
};
class blabla2
{
public:
int y;
blabla2(int a)
{
y=a;
}
};
int main()
{
using namespace boost::adaptors;
std::vector<blabla> test;
auto foo = [](const blabla& A) -> std::pair<blabla2, double> {
return std::make_pair(blabla2(A.x), double(A.x));
};
const auto histogram = test | boost::adaptors::transformed(foo);
// std::vector<std::pair<blabla2, double>> should be return type?
std::vector<std::pair<blabla2, double>> histogramtest = histogram; ----> this line gives error unexpectedly. Why is it so?
std::pair<blabla2, double> x = histogram[0];
}
The line std::vector<std::pair<blabla2, double>> histogramtest = histogram; gives error
While std::pair<blabla2, double> x = histogram[0]; works correctly. Why is that so?

Answer 1

The return value is boost::transformed_range<decltype(foo), std::vector<blabla>>, not std::vector<std::pair<blabla2, double>>. If you want to achieve expected type, you should do something like that:
std::vector<std::pair<blabla2, double>> histogramtest;
boost::copy( test | transformed(foo), std::back_inserter(histogramtest));

Answer 2

You need to copy the range into a vector.
e.g.
#include "boost/range/adaptor/transformed.hpp"
#include "boost/range/algorithm.hpp"
#include <iostream>
#include <vector>
class blabla
{
public:
int x;
};
class blabla2
{
public:
int y;
blabla2(int a)
{
y = a;
}
};
int main()
{
std::vector<blabla> test = { {1}, {2}, {3} };
auto foo = [](const blabla& A) -> std::pair<blabla2, double> {
return std::make_pair(blabla2(A.x), double(A.x));
};
const auto test_range = test | boost::adaptors::transformed(foo);
std::vector<std::pair<blabla2, double>> test_output_vector;
boost::range::copy(test_range, std::back_inserter(test_output_vector));
for (const auto& [b, v] : test_output_vector) {
std::cout << b.y << ", " << v << "\n";
}
}