accessing vectors embedded in class objects

[Popping mad]

I wanted to embed a vector in my class then access it through a wrapper within the
class that involved the subscript operator operator[]<> and I've run across an
unexpected problem I didn't anticipate

So a Class looks like this, with the data member _ofthings<T> being a vector type.

It has an accessory function ofthings() that returns the vector as an available lvaue

T& ofthings(){
return _ofthings;
};

the overload is vissible and when I compile I get an error
|| g++ -Wall -ggdb -pg -M *.c >make.deps
|| g++ -Wall -ggdb -pg -pg -pthread -o lvalue.exe lvalue.c
|| lvalue.c: In function ‘int main(int, char**)’:
lvalue.c|91 col 14| error: assignment of read-only location ‘test2.vect::Lvalue<T>::operator[]<int>(j)’
|| test2[j] = i;
|| ^
|| lvalue.c: In instantiation of ‘const T& vect::Lvalue<T>::operator[](int) [with T = int]’:
lvalue.c|91 col 10| required from here
lvalue.c|57 col 23| error: subscripted value is neither array nor pointer
|| return ofthings()[index];
|| ~~~~~~~~~~^
|| lvalue.c: In instantiation of ‘T& vect::Lvalue<T>::ofthings() [with T = int]’:
lvalue.c|57 col 21| required from ‘const T& vect::Lvalue<T>::operator[](int) [with T = int]’
lvalue.c|91 col 10| required from here
lvalue.c|44 col 13| error: invalid initialization of reference of type ‘int&’ from expression of type ‘std::vector<int>’
|| return _ofthings;
|| ^~~~~~~~~
make: *** [makefile|5| lvalue] Error 1


namespace vect{

/*
* =====================================================================================
* Class: Lvalue
* Description: testing vector access as an lvalue
* =====================================================================================
*/

template < class T >
class Lvalue
{
public:

// ==================== LIFECYCLE =======================================
Lvalue (){}; /* constructor */
Lvalue (std::vector<T> in): _ofthings{in}{}; /* constructor */
Lvalue ( const Lvalue &other ); /* copy constructor */
~Lvalue (){}; /* destructor */

/* ==================== ACCESSORS ======================================= */
T& ofthings(){
return _ofthings;
};

void ofthings(std::vector<T> const vec){
_ofthings = vec;
};
/* ==================== MUTATORS ======================================= */

/* ==================== OPERATORS ======================================= */

const Lvalue& operator = ( const Lvalue &other ); // assignment operator
const T& operator [] (int index)
{
return ofthings()[index];
};
friend std::ostream &operator << (std::ostream &os, const Lvalue in)
{
for(int i = 0; i < in._ofthings.size(); i++ ){
std::cout << i << "\t";
}
return os;
};

/* ==================== DATA MEMBERS ======================================= */
protected:
std::vector<T> _ofthings;

private:

}; /* ----- end of template class Lvalue ----- */

};


Main is this

int main ( int argc, char *argv[] )
{

std::cout << "TEST INPUT TO VECTOR" << std::endl;
std::vector<int> test {0};
for(int i = 0; i<10; i++ ){
test[i] = i;
std::cout << test[i] << "\t";
}
std::cout << std::endl;
std::cout << "TEST INPUT TO OBJECT" << std::endl;
vect::Lvalue<int> test2 {test};
for(int j=0, i = 10; j<10; i--, j++){
test2[j] = i;
std::cout << test2[j] << "\t";
}
std::cout << std::endl;
std::cout << "__DONE__";



return EXIT_SUCCESS;
} /* ----


So I can not overload [] unless I access a pointer or an array?

[Alf P. Steinbach]

On 29.11.2016 07:21, Popping mad wrote:
> [snip]

Generally, start with the very first error message, and ignore the rest.

Figure out what it means.

Fix that error.

Compile.

Repeat.


Cheers & hth.,

- Alf

[Popping mad]

On Tue, 29 Nov 2016 07:22:54 +0100, Alf P. Steinbach wrote:

>
> On 29.11.2016 07:21, Popping mad wrote:
>> [snip]
>
> Generally, start with the very first error message, and ignore the rest.
>
> Figure out what it means.
>
> Fix that error.

As usually, Alf, that is the WRONG answer. You're very consistent in being wrong all the time.
But I did fix the problem in that I needed to correct the return type to vector<T> and not T.

Good work Alf. WHo knows, in 20 years you might be able to learn how to be helpful or read compiler errors. Neither is likely.

[Alf P. Steinbach]

If this is “Popping mad” then he's suddenly acquired

• very much improved English,
• an ungratefulness and tendency to lie that he's not displayed before, and
• posting via a different server than usual.

I guess he'll clear that up, whether he was impersonated. :)

In any case this was posted by a person who is not very nice.


Cheers!,

_ Alf

[ruben safir]

Alf, your off your rocker, truly.

[Alf P. Steinbach]

As far as I can tell Rubin Safir and Popping Mad are the same person.

- Alf