Namespaces
The format of namespaces is:
namespace identifier
{
entities
}
Where identifier is any valid identifier and entities is the set of classes, objects and functions that are included within the namespace. For example:
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In this case, the variables a and b are normal variables declared within a namespace called myNamespace. In order to access these variables from outside the myNamespace namespace we have to use the scope operator ::. For example, to access the previous variables from outside myNamespace we can write:
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The functionality of namespaces is especially useful in the case that there is a possibility that a global object or function uses the same identifier as another one, causing redefinition errors. For example:
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In this case, there are two global variables with the same name: var. One is defined within the namespace first and the other one in second. No redefinition errors happen thanks to namespaces.
using
The keyword using is used to introduce a name from a namespace into the current declarative region. For example:1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 |
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Notice how in this code, x (without any name qualifier) refers to first::x whereas y refers to second::y, exactly as our using declarations have specified. We still have access to first::y and second::x using their fully qualified names.
The keyword using can also be used as a directive to introduce an entire namespace:
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In this case, since we have declared that we were using namespace first, all direct uses of x and y without name qualifiers were referring to their declarations in namespace first.
using and using namespace have validity only in the same block in which they are stated or in the entire code if they are used directly in the global scope. For example, if we had the intention to first use the objects of one namespace and then those of another one, we could do something like:
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Namespace alias
We can declare alternate names for existing namespaces according to the following format:
namespace new_name = current_name;