Compilation errors calling find_if using a functor
- by Jim Wong
We are having a bit of trouble using find_if to search a vector of pairs for an entry in which the first element of the pair matches a particular value. To make this work, we have defined a trivial functor whose operator() takes a pair as input and compares the first entry against a string.
Unfortunately, when we actually add a call to find_if using an instance of our functor constructed using a temporary string value, the compiler produces a raft of error messages. Oddly (to me, anyway), if we replace the temporary with a string that we've created on the stack, things seem to work.
Here's what the code (including both versions) looks like:
typedef std::pair<std::string, std::string> MyPair;
typedef std::vector<MyPair> MyVector;
struct MyFunctor: std::unary_function <const MyPair&, bool>
{
explicit MyFunctor(const std::string& val)
: m_val(val) {}
bool operator() (const MyPair& p)
{
return p.first == m_val;
}
const std::string m_val;
};
bool f(const char* s)
{
MyFunctor f(std::string(s)); // ERROR
// std::string str(s);
// MyFunctor f(str); // OK
MyVector vec;
MyVector::const_iterator i = std::find_if(vec.begin(), vec.end(), f);
return i != vec.end();
}
And here's what the most interesting error message looks like:
/usr/include/c++/4.2.1/bits/stl_algo.h:260: error: conversion from ‘std::pair, std::allocator , std::basic_string, std::allocator ’ to non-scalar type ‘std::string’ requested
Because we have a workaround, we're mostly curious as to why the first form causes problems. I'm sure we're missing something, but we haven't been able to figure out what it is.
