Robinson Robinson - 3 months ago 14
C++ Question

std::find across a set of shared_ptr

I'm sure I'm doing something stupid here, but I can't see it. Why doesn't the following compile?

#include <algorithm>
#include <memory>
#include <vector>
#include <string>

// A class to play with. Encapsulates a name.
class StringClass
{
public:
StringClass(std::string const & name) : MyName(name)
{
}

std::string const & Name() const
{
return MyName;
}

private:
std::string MyName;
};

// The set of instances of "StringClass".

std::vector<std::shared_ptr<StringClass>> MyInstances;

// Function returns "true" if a class with the given name exists in the collection.
bool Exists(std::string const & name)
{
auto i = std::find(MyInstances.begin(), MyInstances.end(), [&](std::shared_ptr<StringClass> const & instance) {
return instance->Name() == name;
});

return i != MyInstances.end();
}


I've made a vector of shared_ptr to a class. The class has a Name() property. All I want to do is iterate the vector looking for an instance of the class with the given name. However, it doesn't compile :(.

Errors are:


1>ClCompile: 1> test.cpp 1>c:\program files (x86)\microsoft visual
studio 10.0\vc\include\algorithm(41): error C2679: binary '==' : no
operator found which takes a right-hand operand of type 'const
anonymous-namespace'::<lambda0>' (or there is no acceptable
conversion) 1> could be 'built-in C++
operator==(std::_Bool_type, std::_Bool_type)' 1> c:\program
files (x86)\microsoft visual studio 10.0\vc\include\exception(470): or
'bool std::operator ==(const std::_Exception_ptr &,const
std::_Exception_ptr &)' 1> c:\program files (x86)\microsoft
visual studio 10.0\vc\include\exception(475): or 'bool
std::operator ==(std::_Null_type,const std::_Exception_ptr &)' 1>

c:\program files (x86)\microsoft visual studio
10.0\vc\include\exception(481): or 'bool std::operator ==(const std::_Exception_ptr &,std::_Null_type)' 1> c:\program files
(x86)\microsoft visual studio 10.0\vc\include\system_error(408): or

'bool std::operator ==(const std::error_code &,const
std::error_condition &)' 1> c:\program files (x86)\microsoft
visual studio 10.0\vc\include\system_error(416): or 'bool
std::operator ==(const std::error_condition &,const std::error_code
&)' 1> while trying to match the argument list
'(std::tr1::shared_ptr<_Ty>, const
anonymous-namespace'::)'
1> with 1> [ 1> _Ty=StringClass 1>

] 1> c:\program files (x86)\microsoft visual studio
10.0\vc\include\algorithm(74) : see reference to function template instantiation '_InIt
std::_Find*,
anonymous-namespace'::<lambda0>>(_InIt,_InIt,const
anonymous-namespace':: &)' being compiled 1> with
1> [ 1> _InIt=std::tr1::shared_ptr
*, 1> _Ty=StringClass 1> ] 1> c:\svn\trunk\test\test\test.cpp(55) : see reference to function
template instantiation '_InIt
std::find,
anonymous-namespace'::<lambda0>>(_InIt,_InIt,const
_Ty &)' being compiled 1> with 1> [ 1> _InIt=std::_Vector_iterator<std::_Vector_val<std::tr1::shared_ptr<StringClass>,std::allocator<std::tr1::shared_ptr<StringClass>>>>, 1>

_Myvec=std::_Vector_val<std::tr1::shared_ptr<StringClass>,std::allocator<std::tr1::shared_ptr<StringClass>>>,
1> _Ty=
anonymous-namespace':: 1> ] 1>
stdafx.cpp 1> Generating Code... 1> 1>Build FAILED. 1> 1>Time Elapsed
00:00:00.87
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Answer

You want to use find_if, not find. Find searches for a value, find_if takes a predicate.