Where are variable declarations valid?

if (int a = 1) {
  a = 2;
}
 

quite generally variable declarations in c++ (unlike c) can happen almost anywhere, but I''m sure someone can provide some specifics.

The code above assigns 1 to a and the whole statement is evaluated as an expression returning true, thus a is then assigned 2.

Well I need to have predefined variables, otherwise I get a real performance hit (Since the variables are already declared in a function, I can use an index into an array instead of having to use a hash every time a variable is used).

I know I can''t do something like:

cout << (int a = 1) << endl;

But I can do:

for(int a=0; a<10; a++) cout << "Hello";

So what are the rules for where a variable can be defined?

Where ever you can have a statement....

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