How would one properly do a static_assert
within a constexpr
function? For example:
constexpr int do_something(int x)
{
static_assert(x > 0, "x must be > 0");
return x + 5;
}
This is not valid C++11 code, because a constexpr function must only contain a return statement. I don't think that the standard has an exception to this, although, the GCC 4.7 does not let me compile this code.
This is not valid C++11 code, because a constexpr function must only contain a return statement.
This is incorrect. static_assert
in a constexpr
function are fine. What is not fine is using function parameters in constant expressions, like you do it.
You could throw if x <= 0
. Calling the function in a context that requires a constant expression will then fail to compile
constexpr int do_something(int x) {
return x > 0 ? (x + 5) : (throw std::logic_error("x must be > 0"));
}
This works and is valid C++11 code, because template arguments are compile time only:
template <int x>
constexpr int do_something() {
static_assert(x > 0, "x must be > 0");
return x + 5;
}
I faced with the same problems as you did with constant expressions in C++. There's few clear documentation about constexprs at the moment. And note that there's some known bugs with it in gcc's issue tracker, but your problem seems not to be a bug.
Note that if you declare constexpr functions inside classes, you are not able to use them inside the class. This also seems to be not a bug.
Edit: This is allowed according to the standard: 7.1.3 states
... or a compound-statement that contains only
- null statements,
- static_assert-declarations
- typedef declarations and alias-declarations that do not
define classes or enumerations, - using-declarations,
- using-directives,
- and exactly one return statement
来源:https://stackoverflow.com/questions/8626055/c11-static-assert-within-constexpr-function