Accessing member variables through boost lambda placeholder

问题

I'm trying to print the second member variable of all items in an stl map using a lambda expression

map<int, int> theMap;
for_each(theMap.begin(), theMap.end(), 
         cout << bind(&pair<int, int>::second, _1) << constant(" "));

but this is not compiling. I essentially want to de-reference the placeholder. Any idea what I'm missing here?

Thanks in advance!

回答1:

std::map will add const to its key; this is to prevent messing up the ordering. Your pair should be:

std::pair<const int, int>

Like dirkgently suggests, use the value_type to always get the correct type. The verbosity is alleviated with a typedef:

typedef std::map<int, int> int_map;

int_map::value_type::second

回答2:

Try:

for_each(theMap.begin(), theMap.end(), 
         cout << bind(&map<int, int>::value_type::second, _1) << constant(" "));

来源：https://stackoverflow.com/questions/2323413/accessing-member-variables-through-boost-lambda-placeholder