C++17 feature where the condition of the if statement gets evaluated at compile time. The result will influence which branch will be used. The other branch only needs a correct syntax.
Questions tagged [if-constexpr]
151 questions
0
votes
0 answers
Confusing behavior with std::is_constructible and if constexpr
Probably easiest to explain if I show an example:
#include
#include
class Unconstructible
{
public:
Unconstructible() = delete;
};
int main()
{
if constexpr (std::is_constructible_v)
{
…
Nathan Pierson
- 5,461
- 1
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- 30
0
votes
1 answer
How to match all string types with std::is_same_v
I'm trying to use templates to generify a method. However I'm struggling to match string types like string char * literals. See the following code example:
template
void match_data_type(ValueType value) {
if constexpr…
BullyWiiPlaza
- 17,329
- 10
- 113
- 185
0
votes
0 answers
Best way to fake static_assert inside if constexpr?
As you may know static_assert does not care about if it is inside "active" branch of if constexpr(it always "runs").
I have a hack workaround that uses invalid array dimensions to trigger the error, but it is ugly.
Are there better solutions in…
NoSenseEtAl
- 28,205
- 28
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- 277
0
votes
1 answer
Why is unreachable code in constexpr if evaluated?
I had expected the compiler to allow invalid or error statements in unreachable code involving constexpr if:
#include
#include
struct ret_t;
struct only_declared_t;
auto test = [](auto a) {
if…
Markus W.
- 307
- 3
- 10
0
votes
1 answer
c++17 language extension and std:c++17 flag
I'm confused by different errors I have in Visual Studio 2017 (version 15.9.11):
'if constexpr' is a C++17 language extension
and
language feature 'structured bindings' requires compiler flag '/std:c++17'
I know that adding /std:c++17 flag will…
zompi
- 268
- 1
- 2
- 10
0
votes
1 answer
How can I prevent implicit conversions in std::is_constructible
Let's say, I have a few different classes:
class constructible_from_float {
public:
constructible_from_float(float);
};
class constructible_from_double {
public:
constructible_from_double(double);
};
class constructible_from_long_double…
John Cvelth
- 522
- 1
- 6
- 19
0
votes
2 answers
3 different / same ways of doing N-factorial compile time in C++
I am trying to play with template metaprogramming, constexpr and if constexpr and have come up with 3 different ways of doing a N-recursive / N-factorial operation.
All three examples are some I've found here on SO or by searching on the net - and…
nelion
- 1,712
- 4
- 17
- 37
0
votes
0 answers
Visual-C++ compilation failure if using a class template utilizing if constexpr statements within another class template
I wrote a matrix class template which, amongst other things, includes arithmetic operators which are specialized for certain class template parameters utilizing if constexpr statements. The use of this matrix class within another template class may…
esakaen
- 43
- 3
0
votes
2 answers
Extend object lifetime/scope from a `if constexpr` branch
Say we have the following code
struct MyClass
{
MyClass() = delete; // or MyClass() { }
MyClass(int) { }
void func() { }
};
int main()
{
if constexpr (std::is_default_constructible_v) {
MyClass myObj;
} else {
…
Phil-ZXX
- 2,359
- 2
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- 40
0
votes
3 answers
constexpr condition not constant?
I wrote the following C++17 code:
constexpr bool gDebug = true;
template constexpr const T& Select(const bool pCondition, const T& a, const T& b)
{
if constexpr (pCondition)
{
return a;
}
else
{
return b;
…
Silicomancer
- 8,604
- 10
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- 130
0
votes
2 answers
Why recursive constexpr template value does not compile?
I am defining a way to know the position of the type in a type list, using recursive templates in C++17. I tried two ways : one using constexpr value and one using constexpr function. The second, using if statement, compiles, while the first, using…
Julien Vernay
- 295
- 3
- 13
0
votes
1 answer
MSVS2017 error "expression did not evaluate to a constant" when compile constexpr with lambda
I am using MSVS c++17 and the code below can`t be compiled:
#include
#include
using namespace std;
template
constexpr auto makeIndices(const Pred &pred)
{
if…
AeroSun
- 2,401
- 2
- 23
- 46
0
votes
2 answers
Issue of `if constexpr` with non-placement new
I'm trying to write a utility that invokes either new T{...} or new T(...) based on whether T is an aggregate type. What I have reached so far is as follows. Note that I'm using a macro instead of a function template because of this issue.
#define…
Lingxi
- 14,579
- 2
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- 93
0
votes
2 answers
if constexpr() gives an error in C++17
I have read about constexpr in C++17 using this reference link.
Then, I made C++ program to test constexpr :
#include
int i = 10;
int func()
{
if constexpr (i == 0)
return 0;
else if (i > 0)
return i;
else
…
msc
- 33,420
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0
votes
1 answer
stop visual studio 17 compile when i hit a bad constexpr if branch
Is there any way to force the compiler to fail if a constexpr if branch is not supposed to be hit?
this code below explains it all better than I could:
template
class arithmetic_type
{
if constexpr(number_base == 0 ||…
Evan
- 109
- 6