Can't assign to mapType::const_iterator? (“no operator = matches”)

Question

typedef map<string,int> mapType;
mapType::const_iterator i;

i = find_if( d.begin(), d.end(), isalnum );

at the '=' i am getting the error:

Error:no operator "=" matches these operands

I know that find_if returns an iterator once the pred resolves to true, so what is the deal?

Answer 1

The documentation for std::find_if

We can only guess at the error as you have only provided half the problem.

Assuming d is mapType and the correct version of isalnum

The problem is that the functor is being passed an object to mapType::value_type (which is how the map and all containers store their value). For map the value_type is actually a key/value pair actually implemented as std::pair<Key,Value>. So you need to get the second part of the object to test with isalnum().

Here I have wrapped that translation inside another functor isAlphaNumFromMap that can be used by find_if

#include <map>
#include <string>
#include <algorithm>
// Using ctype.h brings the C functions into the global namespace
// If you use cctype instead it brings them into the std namespace
// Note: They may be n both namespaces according to the new standard.
#include <ctype.h> 

typedef std::map<std::string,int> mapType;

struct isAlphaNumFromMap
{
    bool operator()(mapType::value_type const& v) const
    {
        return ::isalnum(v.second);
    }
};
int main()
{
    mapType::const_iterator i;
    mapType                 d;

    i = std::find_if( d.begin(), d.end(), isAlphaNumFromMap() );

}

Answer 2

If d is a map , then the problem is with your attempted use of isalnum .

isalnum takes a single int parameter, but the predicate called by find_if receives a map::value_type . The types are not compatible, so you need something that adapts find_if to `isalnum. Such as this:

#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
using namespace std;

typedef map<string,int> mapType;

bool is_alnum(mapType::value_type v)
{
    return 0 != isalnum(v.second);
}

int main()
{
    mapType::const_iterator i;

    mapType d;

    i = find_if( d.begin(), d.end(), is_alnum );
}