Compiler error when attempting to access std::map element with operator[]

Question

I am trying to access elements of a map data structure by key, but am getting a compiler error. I have defined my map data structure using typedefs to simplify the syntax for map instantiation. As you can see, the key is of type string and the data are custom GameComponent objects:

typedef map<string, GameComponent*> ComponentMap;
typedef map<string, GameComponent*>::iterator ComponentMapIter;
typedef map<string, GameComponent*>::const_iterator ComponentMapCIter;

In a derived class of GameComponent, I am creating standard Composite pattern methods along with accessors for each unique GameComponent object stored in my map. However, using the array subscript operator to access objects in accessors results in the compiler error:

void Character::add(const string& key, GameComponent* comp)
{
    m_components->insert( make_pair(key, comp) );
}

void Character::remove(const string& key)
{
    m_components->erase(key);
}

Armor* Character::getArmor() const
{
    // ERROR:
    return static_cast<Armor*>(m_components["Armor"]);
}

Weapon* Character::getWeapon() const
{
    // ERROR:
    return static_cast<Weapon*>(m_components["Weapon"]);
}

Attributes* Character::getAttributes() const
{
    // ERROR:
    return static_cast<Attributes*>(m_components["Attributes"]);
}

The output of the compiler error shows an "invalid type" error, which has me scratching my head:

/Users/Dylan/Desktop/RPG/character.cpp: In member function 'Armor* Character::getArmor() const':
/Users/Dylan/Desktop/RPG/character.cpp:66: error: invalid types 'ComponentMap* const[const char [6]]' for array subscript
/Users/Dylan/Desktop/RPG/character.cpp: In member function 'Weapon* Character::getWeapon() const':
/Users/Dylan/Desktop/RPG/character.cpp:71: error: invalid types 'ComponentMap* const[const char [7]]' for array subscript
/Users/Dylan/Desktop/RPG/character.cpp: In member function 'Attributes* Character::getAttributes() const':
/Users/Dylan/Desktop/RPG/character.cpp:76: error: invalid types 'ComponentMap* const[const char [11]]' for array subscript

score 6 · Answer 1 · edited May 23 '17 at 11:53

6

Since operator[] in a std::map is not const, you can't use it inside const methods (on members, of course).

Use at (C++11) or find & iterators pre-C++11.

Related: Why does std::map not have a const accessor?

edited May 23 '17 at 11:53

Community

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1

answered Dec 26 '12 at 18:55

Luchian Grigore

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Nekuromento · Accepted Answer · 2012-12-26T22:01:05.707

It seems that m_components is of type ComponentMap*. When you write m_components["Armor"] compiler interprets that as an access to "Armor"-th element of dynamic array of ComponentMaps, which does not make any sense.

What you want is (*m_components)["some string"]. This will invoke operator[] of ComponentMap, but as Luchian Grigore and Olaf Dietsche mention, std::map::operator[] does not have a const overload, so this will fail too. The only option left is to use find.

The simplified edition will be:

Armor* Character::getArmor() const
{
    return static_cast<Armor*>(m_components->find("Armor")->second);
}

Weapon* Character::getWeapon() const
{
    return static_cast<Weapon*>(m_components->find("Weapon")->second);
}

Attributes* Character::getAttributes() const
{
    return static_cast<Attributes*>(m_components->find("Attributes")->second);
}

This code does not have the same behaviour as your original example and will fail if m_components does not have "Armor", "Weapon" and "Attributes" elements. The closest we can get is to explicitly handle absence of element and return 0 or nullptr if you use C++11.

Final correct C++03 compatible edition:

Armor* Character::getArmor() const
{
    ComponentMapCIter i = m_components->find("Armor");
    if (i != m_components->end())
        return static_cast<Armor*>(i->second);
    return 0;
}

Weapon* Character::getWeapon() const
{
    ComponentMapCIter i = m_components->find("Weapon");
    if (i != m_components->end())
        return static_cast<Weapon*>(i->second);
    return 0;
}

Attributes* Character::getAttributes() const
{
    ComponentMapCIter i = m_components->find("Attributes");
    if (i != m_components->end())
        return static_cast<Attributes*>(i->second);
    return 0;
}

Yes, this is the one I ended up using. Thanks. – dtg Dec 26 '12 at 20:11 — dtg, Dec 26 '12 at 20:11

score 2 · Answer 3 · edited Sep 12 '13 at 19:47

2

getArmor(), getWeapon() and getAttributes() are defined const, but m_components[] might modify m_components. So you must either not define your methods const or use std::map::find instead.

Armor* Character::getArmor() const
{
    auto i = m_components->find("Armor");
    if (i != m_components->end())
        return static_cast<Armor*>(i->second);

    return nullptr;
}

edited Sep 12 '13 at 19:47

answered Dec 26 '12 at 18:56

Olaf Dietsche

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Is `auto` a C++11 feature? It won't work using my version of the GNU compiler, which I think is pre-C++11... – dtg Dec 26 '12 at 19:23
@Dylan Just try it. It is already present with the `-std=c++0x` option. So if you have a reasonable current gcc (at least gcc 4.6), it will work. – Olaf Dietsche Dec 26 '12 at 19:52
@Dylan Just FYI, `auto` is already available with `gcc-4.4 -std=c++0x` (2009-04-21) and `nullptr` is available with `gcc-4.6 -std=c++0x` (2011-03-25). – Olaf Dietsche Dec 26 '12 at 20:24
Thanks, are these provided in specific header files? – dtg Dec 26 '12 at 20:49
1

@Dylan No header file is needed. When you add `-std=c++0x` to your command line (or `-std=c++11` in case of gcc 4.7), `auto` and/or `nullptr` are available. – Olaf Dietsche Dec 26 '12 at 20:53

Compiler error when attempting to access std::map element with operator[]

3 Answers3