How to add valid key without specifying value to a std::map?

问题

I have a std::map, and I would like to add a valid key to iterate over it later, but without giving any value (it will be given later on in the course of the iterations).

This is how I do it for now :

std::vector<std::string> valid_keys;
//Fill... Then :
std::map<std::string, float> map;
for(size_t i = 0 ; i < valid_keys.size() ; ++i) {
    /*I don't want to do that because in fact I don't use a float type*/ 
    map[valid_keys[i]] = 0.f; //<- 
}
//Using :
for(std::map<std::string, float>::iterator it = map.begin() ; it != map.end() ; ++it) {
    it->second = 0; //Dummy
}

How can I do that, please ?

Thanks aforehand.

回答1:

I'm not entirely sure what you mean by "without giving any value" but if you mean without explicitly assigning a value then just do

map[valid_keys[i]];

This still works i.e. it creates a new entry in the map if there was not previously one with that key. The operator[] just returns a refernce to the value so that you can assign a new value to it but remember it's already been default constructed.

If, on the other hand, you mean you want to express that there is no meaningful value and it may or may not subsequently receive a valid value then see @UncleBens` answer.

回答2:

I suppose something that could help you out is Boost.Optional.

#include <boost/optional.hpp>
#include <map>

class CantConstructMe
{
    CantConstructMe() {}
};

int main()
{
    std::map<int, boost::optional<CantConstructMe> > m;
    m[0];
}

The lack of available default constructor is not an issue, by default optional will be empty.

回答3:

/* I don't want to do that because in fact I don't use a float type */

Then instead of std::map use the std::set.

回答4:

It seems like you really want a std::set or std::list of keys, and then iterate over that set later to create your map.

If for some reason this isn't acceptable, you could wrap your float with some type of object that implements operator float(), and various conversion functions for float, but also has a "good()" flag or somesuch.

If your final values are actually floats, you could initialize the values to +/- NaN as a simple placeholder.

I would say boost::optional is a better option for production code, but it depends on your requirements.

来源：https://stackoverflow.com/questions/3577060/how-to-add-valid-key-without-specifying-value-to-a-stdmap

标签

c++

map