Hidden Features of C++? [closed]

Answer 1

The array operator is associative.

A[8] is a synonym for *(A + 8). Since addition is associative, that can be rewritten as *(8 + A), which is a synonym for..... 8[A]

You didn't say useful... :-)

Answer 2

Many know of the identity / id metafunction, but there is a nice usecase for it for non-template cases: Ease writing declarations:

// void (*f)(); // same
id<void()>::type *f;

// void (*f(void(*p)()))(int); // same
id<void(int)>::type *f(id<void()>::type *p);

// int (*p)[2] = new int[10][2]; // same
id<int[2]>::type *p = new int[10][2];

// void (C::*p)(int) = 0; // same
id<void(int)>::type C::*p = 0;

It helps decrypting C++ declarations greatly!

// boost::identity is pretty much the same
template<typename T> 
struct id { typedef T type; };

Answer 3

map::operator[] creates entry if key is missing and returns reference to default-constructed entry value. So you can write:

map<int, string> m;
string& s = m[42]; // no need for map::find()
if (s.empty()) { // assuming we never store empty values in m
  s.assign(...);
}
cout << s;

I'm amazed at how many C++ programmers don't know this.

Answer 4

Most C++ programmers are familiar with the ternary operator:

x = (y < 0) ? 10 : 20;

However, they don't realize that it can be used as an lvalue:

(a == 0 ? a : b) = 1;

which is shorthand for

if (a == 0)
    a = 1;
else
    b = 1;

Use with caution :-)

Answer 5

Not only can variables be declared in the init part of a for loop, but also classes and functions.

for(struct { int a; float b; } loop = { 1, 2 }; ...; ...) {
    ...
}

That allows for multiple variables of differing types.

Answer 6

Putting functions or variables in a nameless namespace deprecates the use of static to restrict them to file scope.