在开关盒内使用std :: map

Question

I have this:

map<string,int> a;
int b;

And i'd like to make this:

switch(b)
{
  case a["someStr1"]:
  someCode1();
  break;

  case a["someStr2"]:
  someCode2();
  break;

  etc.
}

But it doesn't compiles. How to implement this correctly?

Answer 1

switch conditions need to be constants, so what you want to do here is not possible.

You're better off using some if statements.

Answer 2

switch/case are meant for constants (eg, enum , int s etc.).
You can use the map<>::iterator to run through the values and compare with b .

for(map<string,int>::const_iterator it = a.begin(), end = a.end(); it != end; it++)
{
  if(it->second == b)
  {
    ...
    break;
  }
}

This way you can avoid the code duplication for comparison, if your a is large enough.

Also, you can explore the option of replacing for loop with for_each .

Answer 3

You can't. Expression after case in a switch statement must be integral compile-time constant. So a literal ( 42 ), const int variable initialized with a literal ( const int x = 66 ... case x: ) or enum value. And thats about it.

The reason this is so strict is efficiency. Compilers usually create labels for each case and if you know the value for each label at compile time, you can make some nice optimizations that avoid most of the overhead a normal code branching has.

In your case just go with if-else:

if(b == a["someStr1"]) {
    //...
} else if(b == a["someStr2"]) {
    //...
}   // and so on