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Improve constexpr section by providing a small explanation of one of the snippets and that evaluations MAY happen at compile-time.

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Anthony Calandra
2023-02-17 17:03:16 -05:00
parent 40b3823256
commit 0dd38f0bad
2 changed files with 6 additions and 4 deletions
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README.md
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@@ -1612,7 +1612,7 @@ Attributes provide a universal syntax over `__attribute__(...)`, `__declspec`, e
```
### constexpr
Constant expressions are expressions evaluated by the compiler at compile-time. Only non-complex computations can be carried out in a constant expression. Use the `constexpr` specifier to indicate the variable, function, etc. is a constant expression.
Constant expressions are expressions that are *possibly* evaluated by the compiler at compile-time. Only non-complex computations can be carried out in a constant expression (these rules are progressively relaxed in later versions). Use the `constexpr` specifier to indicate the variable, function, etc. is a constant expression.
```c++
constexpr int square(int x) {
return x * x;
@@ -1628,8 +1628,9 @@ int b = square2(2); // mov edi, 2
// call square2(int)
// mov DWORD PTR [rbp-8], eax
```
In the previous snippet, notice that the computation when calling `square` is carried out at compile-time, and then the result is embedded in the code generation, while `square2` is called at run-time.
`constexpr` values are those that the compiler can evaluate at compile-time:
`constexpr` values are those that the compiler can evaluate, but are not guaranteed to, at compile-time:
```c++
const int x = 123;
constexpr const int& y = x; // error -- constexpr variable `y` must be initialized by a constant expression