constexpr is a modifier introduced in C++11, which informs the compiler that the value of a function or variable is known or can be calculated at compile time. As such, it can be used as a constant in places where otherwise it couldn't be.
Questions tagged [constexpr]
2435 questions
57
votes
6 answers
C++ Linker Error With Class static constexpr
I am compiling the following simple program with g++-4.6.1 --std=c++0x:
#include
struct S
{
static constexpr int X = 10;
};
int main()
{
return std::min(S::X, 0);
};
I get the following linker error:
/tmp/ccBj7UBt.o: In…
Travis Gockel
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57
votes
1 answer
Why can't non-static data members be constexpr?
This is valid code:
struct S {
constexpr S(int x, int y): xVal(x), yVal(y) {}
constexpr S(int x): xVal(x) {}
constexpr S() {}
const int xVal { 0 };
const int yVal { 0 };
};
But here I'd really like to declare xVal and yVal…
KnowItAllWannabe
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57
votes
3 answers
When and why would you use static with constexpr?
As a disclaimer, I have done my research on this before asking. I found a similar SO question but the answer there feels a bit "strawman" and didn't really answer the question for me personally. I've also referred to my handy cppreference page but…
void.pointer
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56
votes
6 answers
In C++11 is sqrt defined as constexpr?
In C++11 is std::sqrt defined as constexpr, i.e. can it legally be used from other constexpr functions or in compile-time contexts like array sizes or template arguments? g++ seems to allow it (using -std=c++0x), but I'm not sure I can take that as…
sepp2k
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55
votes
4 answers
Why do we need to mark functions as constexpr?
C++11 allows functions declared with the constexpr specifier to be used in constant expressions such as template arguments. There are stringent requirements about what is allowed to be constexpr; essentially such a function encapsulates only one…
Potatoswatter
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54
votes
5 answers
Why is C++11 constexpr so restrictive?
As you probably know, C++11 introduces the constexpr keyword.
C++11 introduced the keyword constexpr, which allows the user to
guarantee that a function or object constructor is a compile-time
constant.
[...]
This allows the compiler to…
NoSenseEtAl
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54
votes
4 answers
How does the C++ compiler evaluate recursive constexpr functions so quickly?
I've been learning about C++ constexpr functions, and I implemented a constexpr recursive function to find the nth fibonacci number.
#include
#include
#include
#include
#include
constexpr long long…
ahskdjfk
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- 8
54
votes
4 answers
Use of constexpr in header file
Can I have a definition like this in a header file?
constexpr double PI=3.14;
Is there any problem in having this in a header file that would be included to several cpp files?
I am worried that since it says in standard that this constexpr has its…
mans
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52
votes
4 answers
Can't a class have static constexpr member instances of itself?
This code is giving me incomplete type error.
What is the problem? Isn't allowed for a class to have static member instances of itself?
Is there a way to achieve the same result?
struct Size
{
const unsigned int width;
const unsigned int…
nyarlathotep108
- 5,275
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52
votes
3 answers
Why do constant expressions have an exclusion for undefined behavior?
I was researching what is allowed in a core constant expression*, which is covered in section 5.19 Constant expressions paragraph 2 of the draft C++ standard which says:
A conditional-expression is a core constant expression unless it involves one…
Shafik Yaghmour
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51
votes
3 answers
Does specifying constexpr on a constructor automatically make all objects created from it to be constexpr?
Here is my code:
class test {
public:
constexpr test() {
}
constexpr int operator+(const test& rhs) {
return 1;
}
};
int main() {
test t; //constexpr keyword isn't…
Carlos Miguel Colanta
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48
votes
3 answers
How can the compile-time be (exponentially) faster than run-time?
The below code calculates Fibonacci numbers by an exponentially slow algorithm:
#include
#include
#define DEBUG(var) { std::cout << #var << ": " << (var) << std::endl; }
constexpr auto fib(const size_t n) -> long long
{
…
behzad.nouri
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48
votes
5 answers
static constexpr member of same type as class being defined
I would like a class C to have a static constexpr member of type C. Is this possible in C++11?
Attempt 1:
struct Foo {
constexpr Foo() {}
static constexpr Foo f = Foo();
};
constexpr Foo Foo::f;
g++ 4.7.0 says: 'invalid use of incomplete…
ndkrempel
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47
votes
3 answers
How can I get the depth of a multidimensional std::vector at compile time?
I have a function that takes a multidimensional std::vector and requires the depth (or the number of dimensions) to be passed in as a template parameter. Instead of hardcoding this value I would like to write a constexpr function that will take the…
tjwrona1992
- 8,614
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47
votes
1 answer
Get min / max value of a static constexpr array at compile time
Let's say I have an array of integers defined like that:
static constexpr int IntArray[] = {1, 5, 10, 12, 17};
Is there a way to get the minimum or maximum value at compile time?
lucaboni
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