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
3
votes
0 answers
Error case in a series of "if constexpr" statements
I am refactoring a code that previously used SFINAE in C++14 to handle different categories of types (float, ints, strings, etc.). Something like the following:
template
struct S;
template
struct…
sunmat
- 6,976
- 3
- 28
- 44
3
votes
2 answers
Best way to trigger a compile-time error if no if-constexpr's succeed?
I have a long series of if constexpr statements and would like to trigger a compile-time error if none of them succeed.
Specifically, I have an abstract syntax tree whose result I would like to convert to a specific set of types that I might need. …
Charles Ofria
- 1,936
- 12
- 24
3
votes
2 answers
getting type index from a type list in C++
When I try and get the index of a type within a list of types using below, the code compiles and returns the correct value when the else clause is used. However when I skip the else clause and place the return getIndex(x + 1); just after…
user3882729
- 1,339
- 8
- 11
3
votes
0 answers
if constexpr gcc bug
I noticed my code base not longer compiling with gcc.
I was able to reduce the problem to the following
struct bar {
int foo(){return 0;}
};
int foobar() {
if constexpr(true) {
return 0;
} else {
return [](){
…
Dominic Pöschko
- 51
- 1
- 4
3
votes
3 answers
How to make the compiler ignore this if-constexpr that evaluates to false?
I am writing a macro that when used to wrap a function call like so: macro(function()), returns a std::optional, where T is the return value of the function. So far it works, but I run into problems when trying to get it to return a…
Parker Peterson
- 43
- 3
3
votes
1 answer
Does "if constexpr(something false)" ALWAYS omit template instantiation
Will this template function f() be always NOT instantiated?
if constexpr(something false){
//some template function OR function in template class
f();
}
Below is my test (coliru MCVE).
I created fun() that will instantiate E…
cppBeginner
- 1,114
- 9
- 27
3
votes
2 answers
if constexpr vs if with constant
As shown in this question: link, if both of if branches are valid, there's no difference between:
const int foo = 5;
if (foo == 5)
{
...
}
else
{
...
}
and
const int foo = 5;
if constexpr (foo == 5)
{
...
}
else
{
...
}
in terms…
dabljues
- 1,663
- 3
- 14
- 30
3
votes
4 answers
Ternary operator and if constexpr
I have situations where sometimes based on some bool I want to call 2 constexpr functions that return different types and assign it to auto constant.
Unfortunately ternary operator needs types to be "similar".
I have workaround in the code below…
NoSenseEtAl
- 28,205
- 28
- 128
- 277
3
votes
1 answer
From LANGUAGE DESIGN level, why doesn't "if constexpr" decay to "trival if" when condition cannot be deduced at compile-time
As we know, when constexpr function's return value cannot be known at compile-time, it will be delayed to be computed at run-time(IOW, decay to non-constexpr function). This allows us to adhere constexpr to a function freely and need not worry about…
Chen Li
- 4,824
- 3
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- 55
3
votes
1 answer
When does a constexpr static member stop being a constexpr?
I have this snippet.
#include
#include
struct JustStr {
JustStr(const std::string& x) : val(x) {}
static constexpr bool pred = false;
std::string val;
};
template
class C {
private:
T x;
public:
…
fakedrake
- 6,528
- 8
- 41
- 64
3
votes
2 answers
Using constexpr-if in a generic lambda to determine type of a parameter
I have the following problem: I have a class hierarchy with a base class and two sub-classes. I have implemented a resolve_type function that accepts an instance of the base class and a generic lambda (or similar). The function resolves its type and…
maxj
- 45
- 4
3
votes
1 answer
Using constexpr to validate literal parameters in constructor
I started experimenting with constexpr.
What I'm trying to achieve is to validate literal numeric values
provided as ctor parameters.
I started with the following, throwing if constructing MyStruct
with a value <= 4.
constexpr int validate(int v)
{
…
Stefano Azzalini
- 1,027
- 12
- 21
2
votes
1 answer
Function that can receive any of T, T&, and T&& as input, AND also recognize its type?
I want to create a function that correctly recognizes the type of its parameter.
template void test(T&& t){
if constexpr(std::is_same_v){
std::cout<< "= int"<
cppBeginner
- 1,114
- 9
- 27
2
votes
1 answer
Configure CMake to use newest available C++ standard
I'm building a C++ library with CMake. Is it possible to set the C++ standard to the newest released standard supported by the user's compiler? I checked the docs for the variable CXX_STANDARD, but it only shows how to use one specific…
Shawn McAdam
- 51
- 8
2
votes
2 answers
Why is always_false_v required in this situation?
I'm using std::variant to specify the types of properties that an entity in my project may have, and stumbled upon this code from cppreference:
std::visit([](auto&& arg)
{
using T = std::decay_t;
if…
CakePlusPlus
- 943
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