Question

When I tried to compile the following codes in VS2010, error C2678 prompted.

#include <string>
#include <map>
using namespace std;
class test
{
    private:
        map<string, string> data;
    public:
        test(){};
        ~test(){};
    public:
        const string & get(const string & key)const{return data[key];}; //error C2678
        bool set(const string & key, const string & value){data[key]=value;return true;};
};
void main()
{
    const string key="Hello world!";
    const string value="I'm coming!";
    test t;
    t.set(key,value);
    t.get(key);
}

But when I leave it as functions like

#include <string>
#include <map>
using namespace std;
bool set(const string & key, const string & value, map<string, string> & data)
{
    data[key]=value;
    return true;
}
const string & get(const string & key, map<string, string> & data)
{
    return data[key];
}
void main()
{
    const string key="Hello world!";
    const string value="I'm coming!";
    map<string, string> data;
    set(key, value, data);
    get(key;
}

It does compile and runs.

Does anyone know what’s the problem about?

Answer 1

You’ve declared the get member function of your test class as const. But the std::map operator[] is a non const function, so it cannot be called from a const function. Use the find function instead.

The reason operator[] is non const is because if the key does not exist, then it inserts it into the map along with a default constructed value.