A Bite of Lambda in C++11

12 Mar 2019 Languages C/C++ Lambda closure

Upon the strong suggestion from a senior student, I recently studied and practiced a very advanced syntax sugar, Lambda expressions, introduced in C++11. This post talks about the basic of lambda expressions.

Syntax Example

Unlike Python, there is no keyword in C++ for lambda expressions, but a special sequence of braces [], parentheses (), and brackets {}. The full declaration in C++11 is [capture](params) -> ret {body}, as shown in the following example:

The lambda expression is like a function pointer that can be passed as a parameter when invoking a routine. Here comes an example using the lambda expression defined above:

In the C++ Primer Plus book, lambda expression is compared with function pointer, and functor (a struct or class having operator() defined as a member method, so that can be used as a function, PresumGen in above exmaple). Lambda expression has four advantages over the other two, as summaried by the author:

  • proximity
    programmers can find the defination nearby where it is used (even inside a function call), while you cannot nest the definitions of functions.
  • brevity
    it takes less coding to define and use a lambda expression than function pointer (no need to find a good name for the function, being anonymous) and functor (no struct or class header).
  • efficiency
    it is more likely for compiler to choose inline a lambda expression (or functor), but function pointer is an address implies non-inline function.
  • capability
    with capture, lambda expression can have the same scope as where it is defined. For example, [&] provides access to all variables by references, [x] means capturing variable x by copy it value. eq and IterFill in the above example code illustrate the usage of capture. This is very difficult for function pointer or functor to achieve the same effect.

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