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Ralph Zhang
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 Posted: Sun Aug 14, 2005 6:50 pm Post subject: strange behavior of boost::bind |
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In the following program, I try to find a Person object in a map whose
Name() == "Ralph". I use boost::bind and boost::lambda, they both
compiles on my VC7.1, but the behavior is different. Boost::lambda
outputs 234, which is I expected, but boost::bind outputs 0, I don't
know where does this result come from. What's wrong with boost::bind?
#include <iostream>
#include <algorithm>
#include <map>
#include <string>
#include <boost/lambda/lambda.hpp>
#include <boost/lambda/bind.hpp>
//#include <boost/bind.hpp>
using namespace boost::lambda;
//using namespace boost;
class Person
{
public:
Person(){}
Person(const std::string& name) : name_(name){}
std::string& Name()
{ return name_; }
private:
std::string name_;
};
int main()
{
std::map<int, Person> persons;
persons[123] = Person("Amy");
persons[234] = Person("Ralph");
persons[345] = Person("Simon");
persons[456] = Person("Maggie");
std::cout << "Ralph's Id is: " <<
std::find_if( persons.begin(), persons.end(),
bind(&Person::Name,
bind(&std::map
"Ralph" )->first;
}
using boost::lambda, the result is:
Ralph's Id is: 234
but with boost::bind, the result is:
Ralph's Id is: 0
compiler is VC7.1. What happened?
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Peter Dimov
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| Posted: Mon Aug 15, 2005 10:53 am Post subject: Re: strange behavior of boost::bind |
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Ralph Zhang wrote:
[...]
| Quote: | std::string& Name()
{ return name_; }
[...] |
| Quote: | };
int main()
{
std::map<int, Person> persons;
persons[123] = Person("Amy");
persons[234] = Person("Ralph");
persons[345] = Person("Simon");
persons[456] = Person("Maggie");
std::cout << "Ralph's Id is: "
std::find_if( persons.begin(), persons.end(),
bind(&Person::Name,
bind(&std::map
"Ralph" )->first;
}
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I get an error C2439 on this example; the problem is that the inner
bind returns a temporary Person by value, but Person::Name returns a
reference to its member std::string.
bind on a data member was changed to return a value and not a reference
because of similar reference to temporary issues, but it seems clear
now that this change has caused more problems than it solved.
I'll try to fix it for the next release (1.33.1 if there is one, or
1.34), but in the meantime you need to use
bind<Person&>(&std::map<int, Person>::value_type::second, _1)
to get the old reference-returning behavior back.
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
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