Use this tag for questions about the C++ standard library template std::tuple. It represents an ordered, heterogeneous sequence of objects. Also add the tag [c++] when using this tag.
Questions tagged [stdtuple]
403 questions
25
votes
2 answers
How to get reference to an element of a std::tuple?
You can get the value of the nth element of an std::tuple using std::get(tuple). But I need to pass one element of that tuple as reference to a function.
How do I get the reference to an element of a std::tuple?
danijar
- 32,406
- 45
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24
votes
2 answers
How to get N-th type from a tuple?
I want to make a template where I can input an index and it will give me the type at that index. I know I can do this with decltype(std::get(tup)) but I would like to implement this myself. For example, I would like to do this,
typename get
user2030677
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- 4
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- 36
23
votes
3 answers
Why is it not good to use recursive inheritance for std::tuple implementations?
In this question, Howard Hinnant said
Some implementations of std::tuple use recursive inheritance. But the good ones don't. ;-)
Can someone please shed some light on that?
Johannes Schaub - litb
- 496,577
- 130
- 894
- 1,212
22
votes
1 answer
Is returning a 2-tuple less efficient than std::pair?
Consider this code:
#include
#include
std::pair f1()
{
return std::make_pair(0x111, 0x222);
}
std::tuple f2()
{
return std::make_tuple(0x111, 0x222);
}
Clang 3 and 4 generate similar code for both on…
John Zwinck
- 239,568
- 38
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- 436
22
votes
4 answers
Structured binding to replace std::tie abuse
In reading this summary of the c++17 final features I was a bit surprised by the section on structured bindings (emphasis mine):
structured bindings
Until now, there was a known trick to abuse std::tie to assign a tuple or pair to different…
Emerald Weapon
- 2,392
- 18
- 29
20
votes
4 answers
Return value optimization of tuple/tie
I am looking into return value optimization in the case of tuple/ties and the behavior I observe is not as I expected. In the example below I would expect move semantics to kick in, which it does, but there is one copy operation which remains. The…
thorsan
- 1,034
- 8
- 19
18
votes
2 answers
Is an implementation of std::tuple allowed to fail with triggering a derived-to-base conversion for empty class elements?
This code does not compile with GCC4.7
struct A {};
void f(A);
struct B { B(std::tuple); };
void f(B);
int main() {
f(std::make_tuple(A()));
}
Because GCC derives from A to make use of the empty base class optimization. However that causes…
Johannes Schaub - litb
- 496,577
- 130
- 894
- 1,212
18
votes
3 answers
Confused by default constructor description of std::tuple in the ISO C++ Standard
The Standard says that std::tuple has the following member functions
constexpr tuple();
explicit tuple(const Types&...);
Can someone please explain what is supposed to happen for std::tuple<>?
Johannes Schaub - litb
- 496,577
- 130
- 894
- 1,212
17
votes
4 answers
Making `std::get` play nice with SFINAE
std::get does not seem to be SFINAE-friendly, as shown by the following test case:
template
auto foo(C &c) -> decltype(std::get(c)) {
return std::get(c);
}
template
void foo(...) { }
int main() {
…
Quentin
- 62,093
- 7
- 131
- 191
16
votes
3 answers
Why would you explicitly move a forwarding reference?
I'm looking at some code, and I see the following function:
template
static return_t make_return(Args &&... args)
{
// using std::forward will preserve lvalue args as such, but the point of this function
// is to…
Trevor Hickey
- 36,288
- 32
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- 271
16
votes
2 answers
Is a tuple guaranteed to compress empty elements?
The implementations of std::tuple in both libstdc++ and libc++ use (I presume) the empty base class optimisation to compress empty elements:
struct empty {};
static_assert(sizeof(std::tuple) == sizeof(int), ""); // (1)
My question is…
Tristan Brindle
- 16,281
- 4
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- 82
15
votes
2 answers
Why does overloading operator< for std::tuple not seem to work in priority_queue?
Here is the MWE:
#include
#include
#include
using namespace std;
bool operator<(tuple lhs, tuple rhs)
{
return get<1>(lhs) < get<1>(rhs);
}
int main()
{
priority_queue
MKCCT
- 177
- 6
15
votes
2 answers
Structured bindings for your own type that isn’t a struct or a tuple(via public member function)
I am going through the Herb Sutter's
A journey: Toward more powerful and simpler C++ programming
Structure Binding section
In order to understand the concept .Best is to write a program I tried but getting some error
Just want to try how to use…
Hariom Singh
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15
votes
6 answers
Produce std::tuple of same type in compile time given its length by a template argument
In c++, how can I implement a function with an int template argument indicating the tuple length and produce a std::tuple with that length?
E.g.
func<2>() returns std::tuple();
func<5>() returns std::tuple().
yuefengz
- 3,338
- 1
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- 24
15
votes
2 answers
Requirements for std::ignore
C++11 introduces an object called std::ignore:
const /* unspecified */ ignore;
For brevity, let
typedef decltype(std::ignore) T;
From what I can tell, the only requirement for T is that it is CopyAssignable, due to the specification of std::tie…
nknight
- 1,034
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