Return pointer to array

Question

I'm trying to return a pointer to an array for my function prototype.

class NAV                  
{
    string date;
    float nav;
public:
    NAV(const string&);  
};

const int HistorySize = 300;

class MutualFund
{
    string ticker;
    NAV* history[HistorySize];
public:
    MutualFund(const string& ticker_, const string& historyFile);
    ~MutualFund();
    NAV** getArray() const{return history;}
    void report() const;

};

For NAV** getArray() const{return history;}, I'm getting a compiler error:

error: invalid conversion from 'NAV* const*' to 'NAV**' [-fpermissive]

Any ideas?

Do you actually need the pointer to the structure array? Maybe you need just the array and return its reference in getArray() — Joel
– Joel, Commented Mar 6, 2016 at 4:58
FYI the error means that return history returns NAV* const * but you are trying to return it as NAV**. — kfsone
– kfsone, Commented Mar 6, 2016 at 5:23
Did you intend for history to be 300 pointers to NAV? Or is history supposed to be an array of 300 NAVs? — kfsone
– kfsone, Commented Mar 6, 2016 at 5:27
In Visual Studio the error is error C2440: 'return': cannot convert from 'NAV *const [300]' to 'NAV **' — kfsone
– kfsone, Commented Mar 6, 2016 at 5:33

user4581301 · Accepted Answer · 2016-03-06 04:55:26Z

4

In NAV** getArray() const{return history;} const means the programmer promises that calling this function will not cause changes to the state of the MutualFund. By returning a non-const pointer, NAV**, you are opening up the possibility for the state to be changed through use of the returned pointer. The compiler will not allow this and is telling you that it can only return a pointer to constant data: NAV* const*.

edited Mar 6, 2016 at 4:55

answered Mar 6, 2016 at 4:44

user4581301

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2 Comments

Brian Over a year ago

I'm having trouble implementing your solution. When I use const on NAV**, the compiler says invalid conversion from NAV* const* to const NAV**. Is there any way to return the member "history"? If so, what would the history return type be?

user4581301 Over a year ago

The return type is NAV* const*, just like the error message says.

Aiken Drum · Accepted Answer · 2016-03-06 08:05:44Z

0

Your getter is a const method, so during its execution, all data members are considered const as well. That's why the conversion error says it is converting from const to non-const, since your return value is non-const.

edited Mar 6, 2016 at 8:05

answered Mar 6, 2016 at 4:48

Aiken Drum

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2 Comments

kfsone Over a year ago

Nope; you get this casting error unless you do "NAV* const*".

Aiken Drum Over a year ago

@kfsone Ah, quite so, I missed that there was a second * after the const in the error line. I edited that note out. Nevertheless, it's still true that the const method is turning the member const, so taking its address gives a pointer to const data, which is why it's incompatible with the non-const return type. Either the method shouldn't be const, or the return type should be.

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