casting - What exactly is or was the purpose of C++ function-style casts?

Question

I am talking about "type(value)"-style casts. The books I have read pass over them quickly, saying only that they are semantically equivalent to C-style casts, "(type) value", and that they should be avoided. If they mean the same thing an old-style cast does, why were they ever added to the language? Also, because declarations can contain superfluous parentheses, this code: "T x(T(y));" doesn't do what someone intending to use the function-style casts would expect; it declares a function named x accepting a T and returning a T rather than constructing a T variable named x by casting y to a T.

Were they a mistake in the design of the language?

Answer 1

Function style casts bring consistency to primitive and user defined types. This is very useful when defining templates. For example, take this very silly example:

template<typename T, typename U>
T silly_cast(U const &u) {
  return T(u);
}

My silly_cast function will work for primitive types, because it's a function-style cast. It will also work for user defined types, so long as class T has a single argument constructor that takes a U or U const &.

template<typename T, typename U>
T silly_cast(U const &u) {
    return T(u);
}

class Foo {};
class Bar {
public:
    Bar(Foo const&) {};
};

int main() {
    long lg = 1L;
    Foo f;
    int v = silly_cast<int>(lg);
    Bar b = silly_cast<Bar>(f);
}