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6.2

private trim function returns empty string.

Asked by itsmeandnobodyelse in C++ Programming Language

Tags: string, trim

I've written the following three helpers to trim a std::string:

string trimright(string& s, bool& modified)
{
int len = (int)s.length();
while (len > 0 && (unsigned char)s[len-1] <= ' ')
len--, modified = true;
if (modified)
s = s.substr(0, len);
return s;
}

string trimright(string& s)
{
bool modified = false;
return trimright(s, modified);
}

string trimright(const string& s)
{
bool modified = false;
string str = s;
return trimright(str, modified);
}

The idea is to make all the following statements valid:

const string cs = "Hello World "; // or passed as a const argument
string s = trimright(cs); // s = "Hello World";

trimright(s) += "abcde"; // s = "Hello Worldabcde";

bool modified = false;
if (!trimright(s, modified).empty()) // no change on s
...
if (modified) // modified is false
...

int pos = 16;
string as = trimright(s.substr(0, pos)); // as = "Hello Worldabcde";

The last statement goes wrong. 'as' is empty after that.

When debugging it calls

- string trimright(const string& s)
- string trimright(string& s, bool& modified)

what both is ok. In the latter function 'modified' keeps false and the input string was returned unchanged. Then, some temporary strings were deleted and finally the local 'str' in 'string trimright(const string& s)' was empty, so the return value was empty as well.

I think I can get out of that by changing

return trimright(str, modified);

to

str = trimright(str, modified);
return str;

but I would like to know whether the above code should work correctly with a better compiler (currently I have VC7.1), or that I have made an error of reasoning somewhere, or that I made something that was undefined so that one compiler makes it as I expected and some other will do it wrong.

Regards, Alex

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private trim function returns empty string.

Asked by itsmeandnobodyelse in C++ Programming Language

Tags: string, trim

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