CODEWITHSUNDEEP
c++switch-statement
Motti
Motti

Reputation: 114795

Is it legal to switch on a constant in C++?

I was just made aware of a bug I introduced, the thing that surprised me is that it compiled, is it legal to switch on a constant?

Visual Studio 8 and Comeau both accept it (with no warnings). 

switch(42) { // simplified version, this wasn't a literal in real life
    case 1:
        std::cout << "This is of course, imposible" << std::endl;  
}

Upvotes: 4

Views: 886

Answers (6)

pauljwilliams
pauljwilliams

Reputation: 19225

Yes, it's legal. ​​​​​​​​​​​​​​

Upvotes: 2

anon
anon

Reputation:

It's not impossible that switching on a constant makes sense. Consider:

void f( const int x ) {
   switch( x ) {   
   ...
   }
}

Switching on a literal constant would rarely make sense, however. But it is legal.

Edit: Thinking about it, there is case where switching on a literal makes perfect sense:

int main() {
    switch( CONFIG ) {
           ...
    }
}

where the program was compiled with:

g++ -DCONFIG=42 foo.cpp

Upvotes: 18

Nik
Nik

Reputation: 1364

One good reason for this being legal is that the compiler might well be able to resolve the value at compile time, depending on what stage of development you're at.

E.g. you might use something like this for debugging stuff:

int glyphIndex;
...
#if CHECK_INVALID_GLYPH
glyphIndex = -1;
#endif

switch (glyphIndex)
    ...

The compiler knows for certain that glyphIndex is -1 here, so it's as good as a constant. Alternatively, you might code it like this:

#if CHECK_INVALID_GLYPH
const int glyphIndex = -1;
#else
int glyphIndex = GetGlyph();
#endif

You wouldn't really want to have to change the body of your switch statement just so you could make little changes like this, and the compiler is perfectly capable of rationalising the code to eliminate the parts that will never be executed anyway.

Upvotes: 3

graham.reeds
graham.reeds

Reputation: 16486

Yes, but why you'd want to (unless debugging) is another matter.

It's similar to if (0) or while (true).

Upvotes: 2

ChrisF
ChrisF

Reputation: 137178

Not everything that makes sense to the compiler makes sense!

The following will also compile but makes no sense:

if (false)
{
   std::cout << "This is of course, imposible" << std::endl;
}

It's up to us as developers to spot these.

Upvotes: 16

sharptooth
sharptooth

Reputation: 170519

Yes, it's perfectly legal to switch on any integer expression. It's the same as switching on an integer value returned by a function - a construct used quite often.

Upvotes: 2

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