Why can't I use a std::tuple in a constexpr lambda function

问题

I have the following code:

#include <string_view>
#include <array>
#include <tuple>

struct Variable
{
  size_t index;
  std::string_view name;
  std::tuple<float, float> bounds;
};

constexpr std::array<Variable, 3> myarray = [](){
    std::array<Variable, 3> res{};
    std::array<std::string_view, 3> strings = {"myvar1", "myvar2", "myvar3"};
    std::array<std::tuple<float, float>, 3> bounds = {{{0,1}, {1,2}, {2,3}}};

    for (std::size_t i = 0; i != res.size(); ++i) {
        res[i] = {i, strings[i], bounds[i]};
    }
    return res;
}();

but this code does not compile due to the std::tuple. What is the reason I can't use std::tuple inside a lambda function?

I'm using

c++ -Wall -Winvalid-pch -Wnon-virtual-dtor -Wextra -Wpedantic -std=c++17 -g -o main.o -c main.cpp

to compile the code.

Version of the compiler is: gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)

The error I get is:

../main.cpp:53:3: error: call to non-constexpr function ‘<lambda()>’
 }();
   ^
../main.cpp:44:51: note: ‘<lambda()>’ is not usable as a constexpr function because:
 constexpr std::array<Variable, num_vars> xrt = [](){
                                               ^
../main.cpp:51:39: error: call to non-constexpr function ‘Variable& Variable::operator=(Variable&&)’
     res[i] = {i, strings[i], bounds[i]};
                                   ^
../main.cpp:16:8: note: ‘Variable& Variable::operator=(Variable&&)’ is not usable as a constexpr function because:
 struct Variable
        ^~~~~~~~

回答1:

Neither tuple nor pair have constexpr assignment in C++17.

But even a trivial struct containing pair of values would do the job. You may want to implement own constexpr-compatible structure if required. Trivial version without fluff you need:

struct Couple {
  float a, b;  
};

struct Variable
{
  size_t index;
  std::string_view name;
  Couple bounds;
};

constexpr std::array<Variable, 3> myarray = [](){
    std::array<Variable, 3> res{};
    std::array<std::string_view, 3> strings = {"myvar1", "myvar2", "myvar3"};
    std::array<Couple, 3> bounds = {{{0,1}, {1,2}, {2,3}}};

    for (std::size_t i = 0; i != res.size(); ++i) {
        res[i] = {i, strings[i], bounds[i]};
    }
    return res;
}();

It's possible to arrangle code the way it would use tuple for future standard

回答2:

std::tuple's assignment operators are not constexpr until c++20. If you avoid assignment operators and construct your tuples in-place then your code compiles:

constexpr std::array<Variable, 3> myarray = [](){
    std::array<std::string_view, 3> strings = {"myvar1", "myvar2", "myvar3"};
    std::array<std::tuple<float, float>, 3> bounds = {{{0,1}, {1,2}, {2,3}}};

    std::array<Variable, 3> res { {
        {0, strings[0], bounds[0]},
        {1, strings[1], bounds[1]},
        {2, strings[2], bounds[2]}
    } };
    return res;
}();

回答3:

Since C++14, std::tuple's constructors are constexpr (at least the ones not accepting allocators), so you could use one of those instead of the assignment operator.

Start adding a constexpr constructor to your class

struct Variable
{
    size_t index{};
    std::string_view name{};
    std::tuple<float, float> bounds{};

    constexpr Variable(size_t i, std::string_view str)
        : index{i}, name{str}, bounds{i, i + 1} {}
};

Then, you could construct the array using a couple of template functions taking advantage of std::integer_sequence.

template <class Element, std::size_t... I, typename... ArgsType>
constexpr auto make_array_with_indices_impl(std::index_sequence<I...>, ArgsType... args)
{
    return std::array<Element, sizeof...(args)>{
        Element(I, args)...
    };    
}

template <class Element, typename... ArgsType>
constexpr auto make_array_with_indices(ArgsType... args)
{
    return make_array_with_indices_impl<Element>(
        std::index_sequence_for<ArgsType...>{}, args...
    );
}

Here an example of their usage.

来源：https://stackoverflow.com/questions/60071381/why-cant-i-use-a-stdtuple-in-a-constexpr-lambda-function

标签

c++

c++17

stdtuple