Why can std:set (with a single colon) compile?

I accidentally wrote

std::set<string> keys;

as:

std:set<string> keys;

but weirdly enough, Visual Studio 2013 still compiles.

Why does this happen?

Actually keys is not only defined, but later used as a set of strings, such as

if(keys.find(keystr)==keys.end()){
    keys.insert(keystr);
    toret.push_back(tempv);
}

At block scope, an identifier followed by a single colon introduces a label. Thus, your statement is equivalent to:

set<string> keys;

except that it bears the label std and can be jumped to by the statement goto std;.

For some reason, the name set is known to the compiler---perhaps you did using namespace std;, or using std::set;, or something like that, or perhaps you defined your own set type somewhere.

In the second case, std is a label. It is the same as spelling default incorrectly in a case statement.