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cpp-lambda

admin 11月 12, 2020 0

Lambda Function

construct a closure: an unnamed function object capable of capturing variables in scope.

Syntax

[ capture-list ] ( params ) { body }


[ capture-list]:Capture list can be passed as follows (see below for the detailed description):
                   1.[a,&b] where a is captured by value and b is captured by reference.
                2.[this] captures the this pointer by value
                3.[&] captures all automatic variables odr-used in the body of the lambda by reference
                4.[=] captures all automatic variables odr-used in the body of the lambda by value
                5.[] captures nothing

Delegates and Lambda

What is delegate?
now let’s look at an example:

  #include <functional>
#include <string>
class EmailProcessor
{
public:
    void receiveMessage (const std::string& message)
    {
        if ( _handler_func ) 
        {
            _handler_func( message );
        }
    }
    void setHandlerFunc (std::function<void (const std::string&)> handler_func)
    {
        _handler_func = handler_func;
    }
private:
        std::function<void (const std::string&)> _handler_func;
};

let’s say we want another class that keeps track of the longest message received so far:

class MessageSizeStore
{
    public:
    MessageSizeStore () : _max_size( 0 ) {}
    void  (const std::string& message ) 
    {
        const int size = message.length();
        if ( size > _max_size )
        {
            _max_size = size;
        }
    }
    int getSize ()
    {
        return _max_size;
    }
private:
    int _max_size;
};

with lambda functions we can easily bind var size_store into the function passed into setHandlerFunc

EmailProcessor processor;
MessageSizeStore size_store;
processor.setHandlerFunc( 
        [&] (const std::string& message) { size_store.checkMessage( message ); } 
);