C++ – the point of make_heap

clanguage-designstl

Can someone please tell me the point of the STL heap function templates like std::make_heap? Why would anyone ever use them? Is there a practical use?

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Your direct question would be well-answered by a class in algorithms and data structures. Heaps are used all over the place in algorithms in computer science. To quote from the make_heap function linked below, "a heap is a tree where each node links to values not greater than its own value." While there are lots of applications for a heap, the one that I use most frequently is in search problems when you want to keep track of a sorted list of N values efficiently.

I had similar confusion to yours when I first encountered the STL heap functions. My question was a little bit different though. I wondered "Why isn't the STL heap in the same class of data structures as std::vector?" I thought that it should work like this:

std::heap< int > my_heap;
my_heap.heap_insert( 7 );
my_heap.heap_insert( 3 );

The idea behind the STL heap functions though is that they allow you to make a heap data structure out of several different underlying STL containers, including std::vector. This can be really useful if you want to pass around the container for use elsewhere in your programs. It's also a little bit nice, because you can choose the underlying container of your heap if you so choose to use a something other than std::vector. All you really need are the following:

template <class RandomAccessIterator>
  void make_heap ( RandomAccessIterator first, RandomAccessIterator last );

This means that you can make lots of different containers into a heap A comparator is also optional in the method signature, you can read more about the different things that you can try in the STL pages for the make_heap function.

Links:

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C++ – the difference between #include and #include “filename”

In practice, the difference is in the location where the preprocessor searches for the included file.

For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.

For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.

A more complete description is available in the GCC documentation on search paths.

C++ – What are the differences between a pointer variable and a reference variable in C++

A pointer can be re-assigned:

int x = 5;
int y = 6;
int *p;
p = &x;
p = &y;
*p = 10;
assert(x == 5);
assert(y == 10);

A reference cannot be re-bound, and must be bound at initialization:

int x = 5;
int y = 6;
int &q; // error
int &r = x;

A pointer variable has its own identity: a distinct, visible memory address that can be taken with the unary & operator and a certain amount of space that can be measured with the sizeof operator. Using those operators on a reference returns a value corresponding to whatever the reference is bound to; the reference’s own address and size are invisible. Since the reference assumes the identity of the original variable in this way, it is convenient to think of a reference as another name for the same variable.
```
int x = 0;
int &r = x;
int *p = &x;
int *p2 = &r;

assert(p == p2); // &x == &r
assert(&p != &p2);
```

You can have arbitrarily nested pointers to pointers offering extra levels of indirection. References only offer one level of indirection.

int x = 0;
int y = 0;
int *p = &x;
int *q = &y;
int **pp = &p;

**pp = 2;
pp = &q; // *pp is now q
**pp = 4;

assert(y == 4);
assert(x == 2);

A pointer can be assigned nullptr, whereas a reference must be bound to an existing object. If you try hard enough, you can bind a reference to nullptr, but this is undefined and will not behave consistently.

/* the code below is undefined; your compiler may optimise it
 * differently, emit warnings, or outright refuse to compile it */

int &r = *static_cast<int *>(nullptr);

// prints "null" under GCC 10
std::cout
    << (&r != nullptr
        ? "not null" : "null")
    << std::endl;

bool f(int &r) { return &r != nullptr; }

// prints "not null" under GCC 10
std::cout
    << (f(*static_cast<int *>(nullptr))
        ? "not null" : "null")
    << std::endl;

You can, however, have a reference to a pointer whose value is nullptr.

Pointers can iterate over an array; you can use ++ to go to the next item that a pointer is pointing to, and + 4 to go to the 5th element. This is no matter what size the object is that the pointer points to.
A pointer needs to be dereferenced with * to access the memory location it points to, whereas a reference can be used directly. A pointer to a class/struct uses -> to access its members whereas a reference uses a ..
References cannot be put into an array, whereas pointers can be (Mentioned by user @litb)
Const references can be bound to temporaries. Pointers cannot (not without some indirection):
```
const int &x = int(12); // legal C++
int *y = &int(12); // illegal to take the address of a temporary.
```
This makes const & more convenient to use in argument lists and so forth.

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C++ – the difference between #include and #include “filename”

C++ – What are the differences between a pointer variable and a reference variable in C++

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