Why does BOOST_FOREACH not work sometimes with C++ strings?

问题

I am using BOOST_FOREACH to iterate through the characters of a C++ string like this:

void foobar(const string& str)
{
    BOOST_FOREACH(const char ch, str)
    {
        // Do something with ch
    }

    return;
}

This piece of code works fine with the following compilation modes:

Multi-threaded (Release) (/MT)
Multi-threaded Debug (/MTd)
Multi-threaded DLL (Release) (/MD)

It causes runtime errors (exceptions) only in this mode:

Multi-threaded Debug DLL (Release) (/MDd)

There are no compilation errors or warnings with the above code snippet, leading me to believe that BOOST_FOREACH knows the container it is handling here. Also, changing const char ch to const char& ch has no change in the behaviour.

Why is this code causing this bad runtime behaviour?

Why only in the Debug DLL mode?

Is this usage of BOOST_FOREACH on C++ strings wrong?

If yes, what is the best workaround for it?

(Note that I am working with Visual Studio 2008 and Boost 1.39.)

回答1:

Usage of BOOST_FOREACH on C++ strings is absolutely correct (see http://www.boost.org/doc/libs/1_39_0/doc/html/foreach.html#foreach.introduction).
Looks like the issue in

  
// Do something with ch

回答2:

You should give us more information about your code, because:

Your problem is tied with the VC++ runtime used
As plainly answered by Dmitriy, your problem is most probably caused by the body of the loop

Anyway, with the little info you gave us, I could speculate the following:

The fact the problem happens on debug and not on release is perhaps because a debug check discovered an error, memory corruption, whatever.
The fact it happens only when you switch runtime, with STL code is perhaps you are mixing code from different modules, each one compiled with a different runtime

Of course, the fact your iterating over a const string means nothing should get modified, but as I was unable to reproduce your bug (pun intended), it is difficult to offer a definitive answer.

If you want more info, you need to provide us with the following info:

Is the string object coming from another module (another DLL, another LIB, another EXE), possibly compiled with another runtime ?
If you write the code by hand (using a plain old "for"), does it work ?
What is the exact error message ?

来源：https://stackoverflow.com/questions/1275852/why-does-boost-foreach-not-work-sometimes-with-c-strings

标签

c++

visual-studio

string

boost