As the error message states, the case expressions must be constant. The compiler builds this as a very fast look-up table at compile time and it can’t do that if there is a possibility that the values could change as the program runs.
If you do need them to be variable, not constant, your best bet is to use if/else statements instead.
方法一：
The
case
statements require integral value which must be known at compile-time, which is what is meant by
constant
here. But the
const
members of a
class
are not really
constant
in that sense. They’re are simply read-only.
Instead of fields, you can use
enum
:
class ThisClass { public: enum Constants { EXAMPLE_CONSTANT = 10, ANOTHER_CONSTANT = 20 }; };And then you can write,
switch(c) { case ThisClass::EXAMPLE_CONSTANT: //code break; case ThisClass::ANOTHER_CONSTANT: //code break; };I am getting a ‘case expression not constant’ error in a switch statement. However, the header provides a definition for the used constants, and the constructor provides initialisation for them in its initialization list.
Additionally, when I mouse over the “problem” statements it identifies them as constants.
const int ThisClass::EXAMPLE_CONSTANT error expression must have a constant valueThis seems a little counter-intuitive to me. I did some research and found a similar problem that someone else had. They were told that all constants must in fact be initialised in ‘main’ and that this was a limitation of the language. Is this really the case? It seems unlikely.
You need a “real” compile time integer constant.
const
in C++ means read-only, and a const variable can be initialized just like
int y = 0; const int x = y;
, making
x
a read-only copy of the value
y
had at the time of initialization.
With a modern compiler, you can either use
enum
s or
constexpr
s to store (integral) members of compile-time-constness:
class Foo { public: static constexpr int x = 0; enum { y = 1 }; }; int main () { switch (0) { case Foo::x: ; case Foo::y: ; } }