Template argument deduction is a compiler attempt to deduce template arguments when some are omitted.
Questions tagged [template-argument-deduction]
692 questions
5
votes
1 answer
Is it legal to use an unexpanded parameter pack as the type of a template template parameter's non-type template parameter?
gcc and clang disagree about whether the following code should compile:
template
struct tuple {};
template
struct Test;
template <
typename... Types,
template typename... Outer, // XXX
Types...…
colavitam
- 1,312
- 1
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5
votes
1 answer
Class template argument deduction - why does it fail here?
Why does the following CTAD attempt fail to compile ?
template struct C { C(T,T) {} };
template <> struct C { C(int) {} };
C c(1); //error: template argument deduction failure
I would have expected that the constructor C(int)…
user1958486
- 571
- 3
- 8
5
votes
1 answer
How to specify template deduction guides for template aliases?
I am trying to make this code compile in VS 2019 (16.10.4) with /std:c++17. The following fails:
namespace my
{
template, class _Allocator = std::allocator<_Key>>
using Set = std::set<_Key,…
Rahul Bhobe
- 4,165
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5
votes
0 answers
Nested template class argument deduction differences between compilers
The below example compiles fine in both gcc and msvc:
template
struct Foo {
template
struct Bar {
Bar(T val):
val_(val)
{
}
T val_;
};
};
auto func() {
…
bjaastad_e
- 691
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- 10
5
votes
2 answers
Why can't I deduce one of the class template arguments?
I have some code here
template
struct foo
{
public:
foo(const funcType& func) : m_func(func) {}
~foo() {}
void m_call() { m_func(); }
private:
const funcType& m_func;
T…
Kerem Kalıntaş
- 53
- 1
- 3
5
votes
0 answers
C++17 Deduction Guide defined inside a class is not effective inside this class, but useful outside the class
#include
struct A { };
struct B { };
struct Test
{
template
struct overloaded : Ts { };
// 1st Deduction Guide
template
overloaded(Ts)->overloaded;
// 2nd Deduction Guide for class…
czs108
- 151
- 4
5
votes
4 answers
Why is no deduction for template parameters only used as return type?
If I do not use tempate parameter (type) in a function argument list -> only as return type, then there is no deduction:
template
T zero() { return 0; }
int main()
{
int x = zero();
}
gives:
a.cpp:15:18: error: no matching…
milanHrabos
- 2,010
- 3
- 11
- 45
5
votes
1 answer
How to understand the rules of partial ordering about T& and T const&
template
void show(T&); // #1
template
void show(T const&); // #2
int main()
{
int a = 0;
show(a); // #1 to be called
}
I'm confused about these partial ordering rules. Here are some quotes:…
xmh0511
- 7,010
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- 36
5
votes
2 answers
How to deduce the return type of a lambda?
I want to mimic Ruby's map() method in C++. I am struggling to figure out the return type automatically:
#include
#include
#include
#include
typedef std::string T2;
template
wolfgang
- 69
- 5
5
votes
2 answers
deduction guides for matrix
I'm looking for deduction guide for initialize matrix.
I've tried to use pack in pack and sizeof..., initializer_list>, custom classes for arrays for construct, but nothing is works...
so, i'm looking for initialize
template…
naufumi
- 65
- 4
5
votes
1 answer
Compiler discrepencies in deduction of template parameter of nested alias template
Consider the following code snippet where we we're trying to deduce the template parameter to the function foobar():
struct Bar { static constexpr auto size = 5; };
template >
struct…
Daisy Sophia Hollman
- 6,046
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- 35
5
votes
0 answers
Template-template, variadic template, and deduction guide: compiler bug?
Consider the following heavily templated code:
// Preamble
#include
-
#include
Vincent
- 57,703
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- 388
5
votes
4 answers
Deduced conflicting types in template pack with reference
I'm working on a program with following structure:
#include
#include
void fun(const std::string &text, int a, int b) { // (1)
std::cout << text << a + b << std::endl;
}
template
void…
Thomas Sablik
- 16,127
- 7
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- 62
5
votes
1 answer
Deducing a const l-value reference from a non-const l-value reference in C++ template
Suppose you have the following pair of functions:
void f(const int&) {
// Do something, making a copy of the argument.
}
void f(int&&) {
// Do the same thing, but moving the argument.
}
They are fairly redundant—the only difference…
Paul Merrill
- 580
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5
votes
2 answers
Template parameter can't be deduced on implicitly constructed argument
I would like to have the following code in c++17:
#include
#include
#include
#include
class Foo;
template
class Bar {
public:
std::function m_fn;
template
…
Javyre
- 53
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- 6