# std::map won't accept any reference as second value

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Hi, I am using gcc 4.0.2. This code generates an error, tracing back through the inheritance heirarchy of std::map, stating that it cannot create a reference. std::map<unsigned int, int &> nodes; If I change it to <int, int> there is no problem. I initially tried this with a RenderNode&, but thought that may be the problem, the fact that it also occurs with an int rules that out. here is a link to what I get out of gcc I could just settle for passing by value, however that would mean a slight structural change. Who has any idea for what I can do? Thanks for any help.

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References cannot be reassigned once they are created, so they cannot be stored in a map (since there would be no way to copy or move them around, or default initialize them). Use pointers or values.

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It wont accept a reference because its not allowed. A reference has to be bound when it is declared. std::map<> is not prepared to handle that requirement.

Why is it important to use a reference? Just use a pointer, it exactly the same except you lose by-value semantics.

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Well i guess this is the solution, but I recently discovered that I was using pointers in everything, C style, and need practice passing by reference.

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Quote:
 Well i guess this is the solution, but I recently discovered that I was using pointers in everything, C style, and need practice passing by reference.
In modern C++, you can almost always (if not always) avoid the use of raw 'C-style' pointers in your own code. However, references aren't always the answer (as pointed out above).

Based on the info you've provided, I'd suggest taking a look at either boost::shared_ptr or boost::ptr_map.

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Quote:
 Original post by speciesUnknownWell i guess this is the solution, but I recently discovered that I was using pointers in everything, C style, and need practice passing by reference.

Using references everywhere they are appropriate is good. However, they are simply not appropriate (or indeed possible) to use in a std::map. There's nothing wrong with using pointers in this case, or if your objects are lightweight enough simple values.

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Quote:
 Original post by speciesUnknownWell i guess this is the solution, but I recently discovered that I was using pointers in everything, C style, and need practice passing by reference.

Using a reference type is not the same as passing by reference.

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Highly compressed explanation: A map stores key/value pairs, and references aren't values.

Consider this code.
int i = 5;int& r = i;

Here, r is just another name for the object known as i. r and i are completely interchangable from here on. From a pure language semantics view, r itself does NOT have a value that "refers to i". r and i ARE the same object.

An implementation may use something like a pointer that points to i and store that as a value in r, or it might optimize the reference away completely. But this is completely implementation defined.

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My rule of thumb for references is use references wherever you can and it makes sense, and no more. For example, my parameters are always either by reference or by const-reference, except for built-in types and arrays, where I will allow by value. As an internal implementation, I'm okay with using pointers where it makes sense. If I know the object it refers to at construction time and it never changes, then the class will also use a reference internally. I actually prefer this in some cases. For a fair amount of classes, allowing assignment doesn't make sense anyway.

In the case of a map, for the reasons stated above (no need to restate them), it can't be done. It's actually one reason I wish C++ had implemented Java-style references and done away with pointers completely. The fact that both exist and you can't fully get rid of pointers, even by policy, is an annoyance.

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Use pointers when we need to make decision from NULL-ness.

Use references when we want the caller to be responsible for passing valid parameters.

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