Template argument deduction is a compiler attempt to deduce template arguments when some are omitted.
Questions tagged [template-argument-deduction]
692 questions
124
votes
1 answer
What are template deduction guides and when should we use them?
The C++17 standard introduces "template deduction guides". I gather they're something to do with the new template argument deduction for constructors introduced in this version of the standard, but I haven't yet seen a simple, FAQ-style explanation…
Tristan Brindle
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87
votes
1 answer
What is a nondeduced context?
I've stumbled over "Why is the template argument deduction not working here?" recently and the answers can be summed up to "It's a nondeduced context".
Specifically, the first one says it's such a thing and then redirects to the standard for…
Shoe
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47
votes
4 answers
Make a function accepting an optional to accept a non-optional?
I'm trying to write syntactic sugar, in a monad-style, over std::optional. Please consider:
template
void f(std::optional)
{}
As is, this function cannot be called with a non-optional T1 (e.g. an int), even though there exists a…
YSC
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47
votes
1 answer
What is the partial ordering procedure in template deduction
Reading the C++11 standard I can't fully understand the meaning of the following statement. Example are very welcome.
Two sets of types are used to determine the partial ordering. For each
of the templates involved there is the original function…
yuan
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43
votes
2 answers
Template default argument loses its reference type
Consider
#include
#include
template
T foo(ARG_T v){
return std::is_reference::value;
}
int main() {
int a = 1;
std::cout << foo(a) << '\n';
std::cout <<…
Bathsheba
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36
votes
2 answers
Do we still need to write the empty angle brackets when using transparent std function objects?
With class template argument deduction we can write:
std::less Fn;
However, G++ 8.2 rejects this code:
#include
#include
#include
int main()
{
std::vector v= { 1, 3, 2, 7, 5, 4…
metalfox
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35
votes
6 answers
Template deduction for function based on its return type?
I'd like to be able to use template deduction to achieve the following:
GCPtr ptr1 = GC::Allocate();
GCPtr ptr2 = GC::Allocate();
instead of (what I currently have):
GCPtr ptr1 = GC::Allocate();
GCPtr ptr2 = GC::Allocate();
My…
Marlon
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30
votes
2 answers
Can C++ deduce argument type from default value?
I tried to write this function with a default template argument:
template
void func(int i1, int i2, A a, B b = 123){
...
}
In my mind I can call it like this: func(1, 2, 3) and compiler should deduce type B as int from…
Dave11ar
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27
votes
3 answers
How can I prevent C++ guessing a second template argument?
I'm using a C++ library (strf) which, somewhere within it, has the following code:
namespace strf {
template
inline auto range(ForwardIt begin, ForwardIt end) { /* ... */ }
template
inline auto…
einpoklum
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25
votes
3 answers
What are std::vector deduction guides in C++17?
I read about deduction guides for std::vector from using cppreference.
Example:
#include
int main() {
std::vector v = {1, 2, 3, 4};
std::vector x{v.begin(), v.end()}; // uses explicit deduction guide
}
So, I have some…
msc
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25
votes
4 answers
Partial ordering with function template having undeduced context
While reading another question, i came to a problem with partial ordering, which i cut down to the following test-case
template
struct Const { typedef void type; };
template
void f(T, typename Const::type*) { cout <<…
Johannes Schaub - litb
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24
votes
4 answers
template argument deduction with strongly-typed enumerations
If I have a normal (weak) enumeration, I can use its enumerated values as non-type template parameters, like so:
enum { Cat, Dog, Horse };
template bool magic(T &t)
{
return magical_traits::invoke(t);
}
and call it…
Useless
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24
votes
3 answers
Why can't unique_ptr's template arguments be deduced?
When you have class template argument deduction available from C++17, why can't you deduce the template arguments of std::unique_ptr? For example, this gives me an error:
std::unique_ptr smp(new D);
That says "Argument list of class template is…
That Guy
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23
votes
1 answer
Error with gcc but not with clang when compiling initializer list containing pointers to template function
The following snippet compiles fine in vc++ and clang++, but fails on gcc (inc 9.2) unless i add an explicit cast. Which compiler is right here?
#include
template
void Task(void) {}
int main()
{
for(auto p_task:…
user7860670
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22
votes
1 answer
How does template argument deduction work when an overloaded function is involved as an argument?
This is the more sophisticated question mentioned in How does overload resolution work when an argument is an overloaded function?
Below code compiles without any problem:
void foo() {}
void foo(int) {}
void foo(double) {}
void foo(int, double)…
Leon
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