C++ – boost::program_options::notify() for

boostc

This question is about the C++ Boost program_options library.

All the tutorials are very clear that I should call notify() on my completed variable map, but I'm not sure what this is actually doing for me. Commenting it out didn't seem to have any effect, and the documentation didn't go into much detail:

http://www.boost.org/doc/libs/1_47_0/doc/html/boost/program_options/notify.html

Other sources suggest that it runs "user-defined" functions. If so, how are those functions registered and what do they do? Might they throw exceptions?

Best Answer

notify() is a member function of value_semantic. It is a hook that is provided so that, once the final value of an option is determined, any action that should be taken with that option can be done automatically and be encapsulated in its own function. This prevents code from having one long function that acts on each of the options. As the possible options grow, that kind of procedural code can get unwieldy.

You can see an example of setting a notify function in the Boost manual:

options_description desc;
desc.add_options()
    ("compression", value<int>()->default_value(10), "compression level")
    ("email", value< vector<string> >()
        ->composing()->notifier(&your_function), "email")
    ;

These declarations specify that default value of the first option is 10, that the second option can appear several times and all instances should be merged, and that after parsing is done, the library will call function &your_function, passing the value of the "email" option as argument.

Related Solutions

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.

Best Answer

Related Solutions

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

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

Related Topic