Avoiding key construction for std::map::find()

Question

Suppose I\'ve got a std::map. std::string can be compared to C strings (const char*) without std::string tempo

Answer 1

If the construction of a string from a literal is really a measured performance bottleneck for you, you can use your own class instead of a std::string that holds either a string or a pointer to a literal. The downside is some extra complication, plus adding the size of a pointer to the elements you're inserting into the container. Note that the value is immutable as required by map, so it's safe to store the results of c_str.

class mystring
{
    std::string  str;
    const char * value;
public:
    mystring() : value(NULL)
    {
    }
    void setString(const std::string & s)
    {
        assert(value == NULL);
        str = s;
        value = str.c_str();
    }
    void setLiteral(const char * s)
    {
        assert(value == NULL);
        value = s;
    }
    bool operator<(const mystring & rhs)
    {
        return strcmp(literal, rhs.literal) < 0;
    }
};

std::map m;
mystring text;
text.setString(some_key);
m.insert(std::make_pair(text, some_data));
// ...
mystring key;
key.setLiteral("Olaf");
m[key] = new_value;