Questions tagged [declval]
29 questions
1
vote
1 answer
Type trait to check if istream operator>> exists for given type
I found this type trait which can be used to check if a certain type T supports operator<<:
template
struct has_ostream_operator_impl {
template
static auto test(V*) -> decltype(std::declval() <<…
ph_0
- 617
- 8
- 27
1
vote
1 answer
How to check copy assignment operator in c++ in noexcept() function
I try to check template class T in function do_math() on the possibility of throwing an exception in the copy assignment operator.
But the following code throws an error:
template
void do_math()
noexcept(noexcept(T(std::declval()))…
Tippa Toppa
- 21
- 1
1
vote
1 answer
What is meant by the statement that std::declval cannot be called?
On the cppreference-page for std::declval it says the following:
Return value
Cannot be called and thus never returns a value.
What does this mean? Surely we call it when we use it?
struct Foo { int func(){} …
JensB
- 839
- 4
- 19
1
vote
0 answers
Why isn't std::declval() an lvalue when std::declval() is?
The first of these two lines fails to compile, the second compiles in MSVC 2017:
// std::cout << sizeof(decltype(++std::declval())) << "\n"; //error. expression must be a modifiable lvalue.
std::cout <<…
TRPh
- 187
- 1
- 9
1
vote
3 answers
Compiler infering the template argument
template
class A
{
public:
A(T &t)
: t_(t){}
T t_;
};
int main()
{
int value;
A a(value);
// what I wish for : A a(value);
// which does not compile "missing template argument before…
Vince
- 3,979
- 10
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- 69
1
vote
2 answers
Can I use declval to Construct an Unused Return?
Let's say I have a templatized function that is going to be specialized, so I really don't care about the base implementation. Can I do something like this:
template
T dummy() {
assert(false);
return declval();
}
When I try…
Jonathan Mee
- 37,899
- 23
- 129
- 288
1
vote
1 answer
Is There a declval for Function Pointers?
I have a function and I need to test whether I can pass an argument of a given type to it. For example:
template
decltype(F(declval{})) foo();
Calling foo() does 2 things:
Sets the return type of foo would have the…
Jonathan Mee
- 37,899
- 23
- 129
- 288
1
vote
2 answers
Use invoke_result with void argument type?
I'm trying to do the following:
struct Unwrapper
{
template
auto operator()(const T& arg, std::enable_if_t, void>* = nullptr) {return arg;}
template
auto operator()(const T& arg,…
haelix
- 4,245
- 4
- 34
- 56
1
vote
1 answer
How to use decltype as the LHS of a larger type expression when compiling under VS2010-VS2015
I've got two versions of code both using decltype and declval. One works and one doesn't. They are included below. I've tested this on VS2017 and below and I get the same results. VS2018 will compile it. GCC and Clang both compile it all.
The error…
bradgonesurfing
- 30,949
- 17
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1
vote
2 answers
Concepts/SFINAE error with typename
I'm trying to make a simply example for myself using new concept syntax. I decided to test if a type had operator() defined, and created a struct to test this using the SFINAE paradigm, but I'm running into type issues. Here is my code:
#include…
Anthony
- 1,015
- 8
- 22
1
vote
1 answer
std::declval and unevaluated expressions
With reference to the following example in Proposing Standard Library Support for the C++ Detection Idiom:
// primary template handles types that do not support pre-increment
template< class, class = void_t<> >
struct
has_pre_increment_member :…
user9196120
- 381
- 3
- 9
0
votes
1 answer
detection idiom and default parameter type match
I can write the following to detect, if an object is serializable.
template
struct is_serializable : std::false_type {};
template
struct is_serializable () <<…
Martin Fehrs
- 792
- 4
- 13
0
votes
1 answer
Zero Initialize a Type
Given a variable of an unsigned integral type: foo lets say I want to do this:
const decltype bar{};
cout << (55834574890LL & ~bar) << endl;
That gives me the expected 42. But now let's say that I want to do away with the bar variable. So…
Jonathan Mee
- 37,899
- 23
- 129
- 288
-1
votes
1 answer
Understanding declval function a little further
I just learn the declval keyword in c++ and I was wondering why in the following code (std::add_rvalue_reference::type).constFunc7() x = 3; is not compiling. Isn't it the same thing as decltype(declvalCustom().constFunc7()) y = 3; with the…
roi_saumon
- 489
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