Algorithms (Modern C++)

 

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The latest version of this topic can be found at Algorithms (Modern C++). For modern C++ programming, we recommend that you use the algorithms in the Standard Template Library (STL). Here are some important examples:

• for_each, which is the default traversal algorithm. (Also transform for not-in-place semantics.)

• find_if, which is the default search algorithm.

• sort, lower_bound, and the other default sorting and searching algorithms.

To write a comparator, use strict < and use named lambdas when you can.

  
auto comp = []( const widget& w1, const widget& w2 )  
      { return w1.weight() < w2.weight(); }  
  
sort( v.begin(), v.end(), comp );  
  
auto i = lower_bound( v.begin(), v.end(), comp );  
  

Loops

When possible, use range-based for loops or algorithm calls, or both, instead of hand-written loops.copy, transform, count_if, remove_if, and others like them are much better than handwritten loops because their intent is obvious and they make it easier to write bug-free code. Also, many STL algorithms have implementation optimizations that make them more efficient.

Instead of old C++ like this:

  
for( auto i = strings.begin(); i != strings.end(); ++i ) {  
  :::  
  :::  
}  
  
auto i = v.begin();  
  
for( ; i != v.end(); ++i ) {  
  if (*i > x && *i < y) break;  
}  
  

Use modern C++ like this:

  
for_each( begin(strings), end(strings), []( string& s ) {  
  :::  
  :::  
} );  
auto i = find_if( begin(v), end(v),  [=](int i) { return i > x && i < y; }  );  
  

Range-based for loops

The range-based for loop is a C++11 language feature, not an STL algorithm. But it deserves mention in this discussion about loops. Range-based for loops are an extension of the for keyword and provide a convenient and efficient way to write loops that iterate over a range of values. STL containers, strings, and arrays are ready-made for range-based for loops. To enable this new iteration syntax for your user-defined type, add the following support:

• A begin method that returns an iterator to the beginning of the structure and an end method that returns an iterator to the end of the structure.

• Support in the iterator for these methods: operator*, operator!=, and operator++ (prefix version).

These methods can be either members or stand-alone functions.

Random Numbers

It's no secret that the old CRT rand() function has many flaws, which have been discussed at length in the C++ community. In modern C++, you don't have to deal with those shortcomings—nor do you have to invent your own uniformly distributed random number generator—because the tools for quickly and easily creating them are available in the STL, as shown in <random>.

See Also

Welcome Back to C++
C++ Language Reference
C++ Standard Library