why is there a "no instance of overloaded function matches the argument list" error when attempting vector push_back?

Question

I am doing an assignment for a C++ class and I am getting an error when using push back with vector. I honestly cannot find any info on why this isnt working and giving me a "no instance of overloaded function" error

 class StudentData {

    class StudyModule {
    private:
        const int studyModuleCode;
        const std::string studyModuleName;
    public:
        StudyModule();
        StudyModule(const StudyModule& copy);
        ~StudyModule();
    };

    private:
        const int studentNumber;
        std::string studentName;
        std::vector<StudyModule*> modules;
    public:
        StudentData();
        StudentData(const StudentData& copy);
        ~StudentData();
        void addModules(const StudyModule& module);
        int howManyModules();
};



void StudentData::addModules(const StudyModule& module)
{
    this->modules.push_back(&module);
}

Pop quiz: those pointers in the vector -- are they pointers to constant or non-constant objects, and are you passing a reference to a constant or a non-constant object to this function? — Sam Varshavchik, Oct 26 '21 at 20:43
Minor quibble: Your function is called `addModules()` (emphasis on plurality), but you can only ever add a single module at a time. — sweenish, Oct 26 '21 at 20:43
Bad Rule of 0/3/5 in `StudyModule`. It should be Rule of 0, but 2 of the (not) required functions are declared. `const` private members are pointless. — sweenish, Oct 26 '21 at 20:45

score 1 · Accepted Answer · edited Oct 26 '21 at 22:52

The function addModules() is declared like this:

void addModules(const StudyModule& module);

that is, its parameter is a reference to const object.

But the vector has a pointer to a non-const object as its template argument:

std::vector<StudyModule*> modules;

Thus, the compiler issues a message for this statement:

this->modules.push_back(&module);

because the expression &module has the type const StudyModule *, ie a pointer to a const object.

Even if you would change the template argument of the vector like this:

std::vector<const StudyModule *> modules;

your approach will be unsafe, because the user can pass a temporary object of type StudyModule to the function. In this case, storing a pointer to this object in the vector will become invalid when the temporary object is destroyed afterwards.

Yes, thank you, that fixed it all :D such as small detail breaking everything like usual :) — Dinnea, Oct 26 '21 at 20:52

why is there a "no instance of overloaded function matches the argument list" error when attempting vector push_back?

1 Answers1