Create subset of class object vector as vector with pointers using predicate?-CodePudding

Is there a standard algorithm that I can use to subset a vector of structs by a vector of pointers to these structs?

E.g.

typedef struct {
    int a;
    char b;
} some_struct_t;

int main()
{
    std::vector<some_struct_t> v = { {10, '1'}, { 5, '2'} };
    std::vector<some_struct_t*> v2;

    std::some_subset_command<some_struct_t, some_struct_t*>(begin(v), end(v), std::back_inserter(v2), [](const somestruct&) { if (somestruct.a == 5) { return &somestruct; }});
}

some_subset_command() should construct the new vector using the back_inserter and the output from the lambda predicate.

How can I do this in the most declarative way?

CodePudding user response：

If you don't mind changing your std::vector<some_struct_t*> v2 to a std::vector<std::reference_wrapper<some_struct_t>> v2 (include <functional>), you could use std::copy_if:

#include <vector>
#include <algorithm>
#include <functional>

typedef struct {
    int a;
    char b;
} some_struct_t;

int main()
{
    std::vector<some_struct_t> v = { {10, '1'}, { 5, '2'} };
    std::vector<std::reference_wrapper<some_struct_t>> v2;
    std::copy_if(begin(v), end(v), std::back_inserter(v2), [](const auto &s) {
        return s.a == 5;
    });
}

More information for std::reference_wrapper at cppreference.

CodePudding user response：

If your compiler supports C 20, then you can use range adaptors:

#include <ranges>
#include <vector>

typedef struct {
    int a;
    char b;
} some_struct_t;

int main()
{
  std::vector<some_struct_t> v = { {10, '1'}, { 5, '2'} };
  auto r = v | std::views::filter([](auto& somestruct) { return somestruct.a == 5; })
             | std::views::transform([](auto& somestruct) { return &somestruct; });       
  std::vector<some_struct_t*> v2(r.begin(), r.end());
}

Demo.