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
2
votes
1 answer
if constexpr Seems to Only Work if Both Cases are Valid
Given the following code:
template
constexpr remove_reference_t has_x() {return true;}
template
constexpr bool has_x(U...) {return false;}
class A { public: int x; };
int main()
{
…
Jonathan Mee
- 37,899
- 23
- 129
- 288
2
votes
1 answer
Why if constexpr fails to bypass constexpr evaluation?
I'm building a static loop for type dispatching using macros. Here is
what I achieved so far.
#define LOOP(n, f) \
static_assert(n <= 8 && "static loop size should <= 8"); \
do { …
Amos
- 3,238
- 4
- 19
- 41
2
votes
2 answers
Instantiating template function in a false if constexpr gives error
Consider the following program:
#include
template
constexpr int constexprValue(Params_t...) { return 5; }
int main()
{
const bool flag = true;
if constexpr(flag)
{
constexpr int value =…
Warp
- 363
- 2
- 9
2
votes
2 answers
"Expected a statement" in constexpr if else expression
I have a function test, which prints out the underlying type of an enum parameter:
enum class TestEnum : uint32_t
{
};
template
void test(TEnum v)
{ // Line 12
if constexpr…
Silverlan
- 2,783
- 3
- 31
- 66
2
votes
1 answer
LLVM coverage confused by if-constexpr
I have encountered a weird problem with LLVM coverage when using constant expressions in an if-statement:
template
int foo(const T &val)
{
int idx = 0;
if constexpr(std::is_trivially_copyable::value && sizeof(T) <=…
Resurrection
- 3,916
- 2
- 34
- 56
2
votes
1 answer
C++ 17: How to call a different constructor using if constexpr?
Say I have a class that takes a boolean in its constructor and depends on the value of the boolean, if calls different functions.
class MyClass {
MyClass(bool is_second)
{
common_code();
if (!is_second)
…
motam79
- 3,542
- 5
- 34
- 60
2
votes
0 answers
Possible VC++17 compiler bug with if constexpr?
While playing around with some of the new C++17 features in Visual C++ - I came across an issue which to me seems like a bug. The code seems to generate as expected when compiling under other compilers using compiler explorer. The error that…
Floris
- 55
- 2
1
vote
3 answers
constexpr switch statements in C++
Do we have a switch constexpr support in C++?
For context, we all know that we have if constexpr for compile-time if statements. Those if-statements will have very low overhead as compiler will evaluate them at compile time. We (or in particular I)…
mgNobody
- 738
- 7
- 23
1
vote
0 answers
compiling/non-compiling discarded if constexpr sub-statements in templates
I was reading cppreference page for If statements, as well as C++17 standard (draft) and I also found out a (non exhaustive) pre-existing question on stackoverflow.
What I understand is that, within a template, a discarded sub-statement of an if…
user20575107
- 59
- 5
1
vote
2 answers
constexpr base case not considered during compilation
I was trying out the constexpr functions and stumbled upon the below example of implementing Fibonacci numbers
There is no logical difference between fibon2 and fibon1 but I still get the compilation error of exceeding template initializations for…
user3703826
- 87
- 1
- 6
1
vote
1 answer
Is it possible to use 'if constexpr' to determine if a template function could be instantiated with a particular type parameter?
Suppose there exists the following code:
class Foo {
public:
void foo() const { std::cout << "foo" << std::endl; }
};
class Bar {
public:
void bar() const { std::cout << "bar" << std::endl; }
};
template
void DoFoo(const T&…
Tyler McHenry
- 74,820
- 18
- 121
- 166
1
vote
0 answers
`if constexpr` with `template` arguments vs. with `constexpr` expressions
Consider the following definition:
template struct Foo;
template<> struct Foo<1> { void fn1(); };
template<> struct Foo<2> { void fn2(); };
Foo<1> has a member function fn1 and similarly Foo<2> has fn2. We can use these functions, selecting…
geometrian
- 14,775
- 10
- 56
- 132
1
vote
2 answers
How can I static assert to disallow "mixed endianness" in a non-templated member function
I am using 2 x std::uint64_t and 1 x std::uint32_t in a high performance implementation of of operator<=> in a struct conataining a std::array.
I am trying to make it cross compiler and architecture compatible.
As part of that I am…
Oliver Schönrock
- 1,038
- 6
- 11
1
vote
1 answer
SFINAE User-Defined-Conversion Operator
I'm trying to do a templated user-defined-conversion that uses design by introspection.
In C++20 I can do the following:
template
operator T() const
{
if constexpr( requires { PFMeta::from_cursor(*this); } )
…
perryperry
- 33
- 10
1
vote
0 answers
Ternary operator works with constexpr expressions but `if constexpr` does
I am playing around in C++17 with clang version 13.0.0 to test whether a given constexpr value is NaN with wrappers but I keep getting errors with if constexpr but the compiler passes using ternary operator
template
constexpr bool…
Kish
- 293
- 1
- 6