(Boost) Why do we need a generic type?

Question

why do we need a generic type specified in Boost library? Isn't template enough? Fore example, if I want a container of a specific type, I would just do:

template<class Type>
vector<Type> v;

If I want to specify a container which contains everything, I simply write:

vector v;

In explanation for boost::any (http://www.boost.org/doc/libs/1_46_1/doc/html/any/s02.html)

Converting types that can hold one of a number of possible value types, e.g. int and string, and freely convert between them, for instance interpreting 5 as "5" or vice-versa. Such types are common in scripting and other interpreted languages. boost::lexical_cast supports such conversion functionality.

Why do we need implicit type like in scripting languages such as PHP?

Further more, in the boost::any example, why:

using boost::any_cast;
typedef std::list<boost::any> many;

void append_int(many & values, int value)
{
    boost::any to_append = value;
    values.push_back(to_append);
}

is acceptable? Does the container use the operator = implemented in boost::any?

any & operator=(const any &);

which makes boost::any able to hold any type? Operator = defined in boost::any is explained as:

Effects: Copies content of rhs into current instance, discarding previous content, so that the new content is equivalent in both type and value to the content of rhs, or empty if rhs.empty().

Throws: std::bad_alloc or any exceptions arising from the copy constructor of the contained type. Assignment satisfies the strong guarantee of exception safety.

http://www.boost.org/doc/libs/1_46_1/doc/html/boost/any.html

Jan Hudec Jan Hudec · Accepted Answer · 2011-05-24T07:05:40

Because C++ does not have generic type. It has type templates, which are compiled for each substituted type separately. The expression

vector v;

is a syntax error, because vector is not a class (it's a class template that can be instantiated to class by giving it template parameters).

There is a somewhat generic type in C++, the void*, but then it's your responsibility to remember what you stored in it (esp. for purposes of deletion). The boost::any is a typesafe alternative that remembers what you stored there and will give an error if you try to convert it to something it does not have defined conversion to (you still have to ask it for the actual value by any_cast).

As for operator=, yes, it is used by the container. The standard containers generally require the element type to be default constructible and assignable, which means they need to have working copy constructor and assignment operator. They generally either don't have to be default constructible or only have to be default constructible if you use certain operations.

(Boost) Why do we need a generic type?

3 Answers