C ++ std :: get< variable>失败了 [英] C++ std::get<variable> fails
问题描述
我如何使用变量使用 std :: get<> 索引到元组?我有以下代码:
How do I use a variable to index into a tuple using std::get<>? I have the following code:
#include <iostream>
#include <tuple>
using namespace std;
int main() {
tuple<int, int> data(5, 10);
for (int i=0; i<2; i++) {
cout << "#" << i+1 << ":" << get<i>(data) << endl;
}
return 0;
}
，它失败，并出现以下编译器错误:
and it fails with the following compiler error:
prog.cpp: In function 'int main()':
prog.cpp:10:39: error: the value of 'i' is not usable in a constant expression
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
prog.cpp:9:11: note: 'int i' is not const
for (int i=0; i<2; i++) {
^
prog.cpp:10:46: error: no matching function for call to
'get(std::tuple<int, int>&)'
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
prog.cpp:10:46: note: candidates are:
In file included from /usr/include/c++/4.9/tuple:38:0,
from prog.cpp:2:
/usr/include/c++/4.9/utility:143:5: note: template<unsigned int _Int,
class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int,
std::pair<_Tp1, _Tp2> >::type& std::get(std::pair<_Tp1, _Tp2>&)
get(std::pair<_Tp1, _Tp2>& __in) noexcept
^
/usr/include/c++/4.9/utility:143:5: note: template argument
deduction/substitution failed:
prog.cpp:10:46: error: the value of 'i' is not usable in a constant
expression
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
prog.cpp:9:11: note: 'int i' is not const
for (int i=0; i<2; i++) {
^
prog.cpp:10:46: note: in template argument for type 'unsigned int'
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
In file included from /usr/include/c++/4.9/tuple:38:0,
from prog.cpp:2:
/usr/include/c++/4.9/utility:148:5: note: template<unsigned int _Int,
class _Tp1, class _Tp2> constexpr typename std::tuple_element<_Int,
std::pair<_Tp1, _Tp2> >::type&& std::get(std::pair<_Tp1, _Tp2>&&)
get(std::pair<_Tp1, _Tp2>&& __in) noexcept
^
/usr/include/c++/4.9/utility:148:5: note: template argument
deduction/substitution failed:
prog.cpp:10:46: error: the value of 'i' is not usable in a constant
expression
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
prog.cpp:9:11: note: 'int i' is not const
for (int i=0; i<2; i++) {
^
prog.cpp:10:46: note: in template argument for type 'unsigned int'
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
In file included from /usr/include/c++/4.9/tuple:38:0,
from prog.cpp:2:
/usr/include/c++/4.9/utility:153:5: note: template<unsigned int _Int,
class _Tp1, class _Tp2> constexpr const typename
std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(const
std::pair<_Tp1, _Tp2>&)
get(const std::pair<_Tp1, _Tp2>& __in) noexcept
^
/usr/include/c++/4.9/utility:153:5: note: template argument
deduction/substitution failed:
prog.cpp:10:46: error: the value of 'i' is not usable in a constant
expression
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
prog.cpp:9:11: note: 'int i' is not const
for (int i=0; i<2; i++) {
^
prog.cpp:10:46: note: in template argument for type 'unsigned int'
cout << "#" << i+1 << ":" << get<i>(data) << endl;
^
In file included from /usr/include/c++/4.9/tuple:38:0,
from prog.cpp:2:
/usr/include/c++/4.9/utility:162:5: note: template<class _Tp, class
_Up> constexpr _Tp& std::get(std::pair<_T1, _T2>&)
get(pair<_Tp, _Up>& __p) noexcept
我实际上截断了编译器错误消息，因为我认为它并没有增加太多.知道如何使它起作用吗?
I actually truncated the compiler error message as I think it does not add much beyond thid point. Any idea how to make that work?
仅需澄清，使用array类型并不是真正的选择.我必须使用tuple，因为它是第三方库中API的返回类型.上面的示例只是为了使其易于理解.
Just to clarify, using an array type is not really an option. I have to use the tuple cause it is the return type of an API from a third party library. The example above is just to make it easy to understand.
推荐答案
如何使用变量使用std :: get<>索引到元组?
How do I use a variable to index into a tuple using std::get<>?
您不知道，std::get<>参数值必须在编译时知道.
You do not, std::get<> parameter value must be known at compile time.
有什么办法使它起作用吗?
Any idea how to make that work?
是，请使用正确的类型:
yes, use proper type:
int main() {
std::array<int, 2> data{ 5, 10 };
for (int i=0; i<2; i++) {
cout << "#" << i+1 << ":" << data[i] << endl;
}
return 0;
}
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