Questions tagged [declval]
29 questions
51
votes
3 answers
Why does std::declval add a reference?
std::declval is a compile-time utility used to construct an expression for the purpose of determining its type. It is defined like this:
template< class T >
typename std::add_rvalue_reference::type declval() noexcept;
Would this not be simpler…
Potatoswatter
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50
votes
2 answers
Why does the implementation of declval in libstdc++-v3 look so complicated?
The code below comes from libstdc++-v3 std::type_traits, which is an implementation of std::declval:
template // template 1
_Up
__declval(int);
template // template 2
_Tp
…
expoter
- 1,622
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18
votes
1 answer
Understanding declval optimized implementation
Looking at libstdc++ source code, I found the following declval implementation:
template
_Up __declval(int); // (1)
template
_Tp __declval(long); // (2)
template
auto declval()…
Igor R.
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- 2
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15
votes
2 answers
How do we test if an expression of a certain type can be invoked with a prvalue?
With c++17 we have fancy new is_invocable and fancy new prvalues that aren't really values.
This permits you to create an object without having to first logically construct it, then elide the construction.
I have run into a problem where using…
Yakk - Adam Nevraumont
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12
votes
2 answers
Does T have to be a complete type to be used in `std::declval`?
Consider this example (coming from here):
#include
#include
template
struct A {
};
struct B {
template
A f() { return A{}; }
using default_return_type =…
463035818_is_not_an_ai
- 109,796
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12
votes
1 answer
std::declval vs crtp, cannot deduce method return type from incomplete type
I am trying to do something like this (in c++11):
#include
template
struct base {
using type = decltype( std::declval().foo() );
};
struct bar : base {
int foo() { return 42;}
};
int main() {
bar::type…
463035818_is_not_an_ai
- 109,796
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8
votes
1 answer
Is it legal to use std::declval in lambda in unevaluated contexts?
Code as below or on godbolt compiles with gcc and MSVC but fails with clang. I couldn't find if/where it is forbidden in the standard. In my opinion it should be supported.
So who is correct on this, clang or gcc/MSVC?
#include
void…
wanghan02
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7
votes
1 answer
std::reference_wrapper, constructor implementation explaination
I have been trying to understand the implementation of std::reference_wrapper, from here, which is as follows:
namespace detail {
template constexpr T& FUN(T& t) noexcept { return t; }
template void FUN(T&&) = delete;
}
…
warrior_monk
- 383
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6
votes
3 answers
Is std::declval outdated because of guaranteed copy elision?
The standard library utility declval is defined as:
template add_rvalue_reference_t declval() noexcept;
To add a rvalue reference here seemed like a good idea, if you think about the language when it was introduced in C++11: Returning a…
Tobi
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5
votes
1 answer
Using declval with a reference type
I see some code examples where the type used to instantiate the std::declval template function is specified as a reference type rather than just a type, as in:
std::declval()
as opposed to:
std::declval()
where T is some type. I am missing…
Edward Diener
- 127
- 3
4
votes
0 answers
How to retrieve the type of a variadic template argument?
I cant find a way to define a type like this:
template
using hasSomeFunc = decltype(std::declval().SomeFunc(std::declval>(),…
Iñigo
- 97
- 6
3
votes
1 answer
For what T does `std::declval()` not have a matching function?
I was surprised to find that for some T, decltype(std::declval()) is not legal:
#include
template
using Alias = decltype(std::declval());
// as expected
using A1 = Alias;
using A2 = Alias;
// error: no…
Eric
- 95,302
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2
votes
0 answers
static assertion on noexcept for protected constructors
I'm attempting to assert following protected class constructors may or may not throw as follows:
#include
#include
class X
{
protected:
X() noexcept(false) { }
X(int) noexcept { }
};
int main()
{
// should fail
…
metablaster
- 1,958
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2
votes
2 answers
How should this c++ typedef using decltype and declval be written to make it portable?
I have the following
template
struct ResultOf {
typedef typename decltype(boost::declval()(boost::declval())) Type;
};
It was written so that VS2010 could have a result_of that worked for a specific use…
bradgonesurfing
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- 17
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1
vote
1 answer
error: use of 'decltype(auto) X before deduction of 'auto' (for generated lambda)
I'm trying to hammer down the type of a this-capturing lambda using a special generating member function with decltype(auto). However, the compiler resists to determine the lambda's type for the following contexts that require it, even though it is…
glades
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