Thread: C++ Map - Error in simple code

Hi

I can't figure out why this short piece is giving an error at the point where I add an element to the map. Any ideas much appreciated, thanks in advance.

Code:

#include <map>
using namespace std;

class component{
int failureRate;
public:
       int getFailrate(){
           return failureRate;
           };
       component(int Ifail):failureRate(Ifail){};
};

int main(){
    component comp(8);
   map<int,component> compMap;

   compMap[8] = comp; // this line is giving a compilation error!
    
    while(1);
    return 0;
}

Because you have no default constructor. Add this line to the component:

Code:

component()
{
}

and remove the ; after method bodies of getFailrate() and component(int Ifail).
Or, you can add the default value for the existing constructor.

Thanks for the help Karas, it now compiles.

Would you be able to explain to me why I need the default constructor, since the component Object had already been initialised when I attempted to add it to the map?

Also why does this code in main() fail :

Code:

int temp = compMap[8].getFailrate;

thanks again

Code:

int temp = compMap[8].getFailrate();  // () added

thanks and apologies as I should of spotted that.

Because you are using the operator= for the class component (you didn't defined any operator= for this class, so compiler did it). operator= requires that class has default constructor.

>> operator= requires that class has default constructor.
This is not true. You can have the operator= without a default constructor.

The problem is with the use of operator[] for the map. The map's operator[] creates a new entry in the map if one does not exist. This is true even if you are attempting to assign a new entry. So in the original code, compMap[8] = comp attempts to insert a new entry with a key of 8 and the value of a default constructed component, then assign the comp variable as a new value for that key.

If your class does not need a default constructor, don't create one just for this problem. Use insert instead of operator[] with the map:

Code:

compMap.insert(std::make_pair(8, comp));