Re: C++ - how to convert string to uppercase/lowercase

James Kanze <>
Thu, 18 Dec 2008 02:04:11 -0800 (PST)
On Dec 17, 10:46 pm, Noah Roberts <> wrote: wrote:

You have to create my_toupper (or use a cast) due to a
C++ wart.

What wart?

To begin with, the fact (inherited from C) that you can't call
toupper with a char without encurring undefined behavior.

#include <cctype>
#include <algorithm>
#include <string>
#include <iostream>

int main()
   std::string x = "hello world";
   transform(x.begin(), x.end(), x.begin(), toupper);
   std::cout << x << std::endl;

Works fine in VS and G++.

Like his original code, it has undefined behavior. Unlike his
original code, there's also a very good chance that it won't
compile. There are two problems: the use of toupper is
ambiguous; this is easily resolved by including <ctype.h>
instead of <cctype>, and specifying ::toupper. The second
problem is more subtle: ::toupper takes an int as argument, not
a char, and it has a pre-condition that the value is in the
range 0...UCHAR_MAX (or EOF). If plain char is signed (as is
all too often the case), calling it with a plain char results in
undefined behavior. (There are other problems with this
solution, e.g. ::toupper uses mutable global state, which may
cause problems in a multithreaded environment. But they don't
apply here.)

And of course, there's the more general problem that transform
can't handle case transformations, because there's not a one to
one mapping of lower case to upper case. But that issue affects
any code which attempts to use any of the toupper functions in
the standard library.

James Kanze (GABI Software)
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