Variable Scope in C++

Question

If I had the following code:

for(int myvar = 0; myvar < 10; myvar++);
if(1)
{
    int var2 = 16;
}

Then, afterwards I wrote the following:

myvar = 0;
var2 = 0;

Would that be legal? My VC++6 compiles it correctly, but I think this should be illegal. (It gives a compiler error in one of my other compilers.)

Answer 1

VC6 is rather old, and not always ... rigid ... in its application of the standard :-) It actually leaked scope in certain circumstances like:

for (int i = 0; i < 10; i++) { }
// You can still use 'i' here.

This led to some funky macro magic to get around this problem. If you're using a ISO-conformant compiler, both those things you try to do are illegal.

From ISO C++11 3.3.3/1, dealing with the introduction of block scope with {...}:

A name declared in a block is local to that block; it has block scope. Its potential scope begins at its point of declaration and ends at the end of its block.

Section 6.5.3 covers the scope of variables "created" by a for statement:

If the for-init-statement is a declaration, the scope of the name(s) declared extends to the end of the for-statement.

Answer 2

No, it would not be (§3.3.2 Local scope):

1. A name declared in a block (6.3) is local to that block. Its potential scope begins at its point of declaration (3.3.1) and ends at the end of its declarative region.

I recommend you use compilers released in the last decade.

Answer 3

VC took about a decade to implement proper scope for variables declared in loops and conditional statements. Generally you can't rely on VC6' judgment regarding C++.