::tolower vs std::tolower difference

Question

I have

using namespace std;
vector<char> tmp;
tmp.push_back(val);
...

Now when I try

transform(tmp.begin(), tmp.end(), tmp.begin(), std::tolower);

It fails to compile, but this compiles:

transform(tmp.begin(), tmp.end(), tmp.begin(), ::tolower);

What's the problem with std::tolower? It works with one argument, e.g., std::tolower(56) compiles. Thanks!

Related/duplicate: http://stackoverflow.com/q/5270780/ (see the second answer http://stackoverflow.com/a/5270970/ ) — dyp, Jul 23 '15 at 14:24
@dyp So if `std::lower` needs two arguments, how come `std::tolower(56)` works? — hovnatan, Jul 23 '15 at 14:32
`std::tolower` is overloaded, there are two functions (one [function from the C library](http://en.cppreference.com/w/cpp/string/byte/tolower) taking one argument and [one function template](http://en.cppreference.com/w/cpp/locale/tolower) which takes two arguments) with the name `tolower` in namespace `std`. In the global namespace, if there is anything, then it's only the C library's `tolower` which takes one argument. — dyp, Jul 23 '15 at 14:34

NathanOliver · Answer 1 · 2015-07-23T14:33:04.110

3

std::tolower has two overloads and it cannot be resolved for the UnaryOperation where the C version ::tolower does not.

If you want to use the std::tolower you can use a lambda as

transform(tmp.begin(), tmp.end(), tmp.begin(), [](unsigned char c) {return std::tolower(c); });

edited Jul 23 '15 at 14:33

answered Jul 23 '15 at 14:22

NathanOliver

3

There also is a unary `std::tolower`, you just need to disambiguate. – Baum mit Augen Jul 23 '15 at 14:23
@BaummitAugen, can it be disambiguated via `&std::tolower`? – Aaron McDaid Jul 23 '15 at 14:28
@AaronMcDaid I don't think so, but you can try if you want to. :) – Baum mit Augen Jul 23 '15 at 14:31
1

I believe disambiguation would be like this: `(int(*)(int)) &std::tolower`. Maybe `(int(int)) std::tolower` would work, but probably not. – celticminstrel Jul 23 '15 at 14:44
@celticminstrel, the former works, with or without the `&`. But I guess the `&` shows the intent more clearly. – Aaron McDaid Jul 23 '15 at 14:49

1 Answers1