C++ meaning of [ ] [duplicate]

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What is a lambda expression in C++11? (10 answers)

Closed 9 years ago.

That's from an example of boosts asio. What does [this] mean? why the []?

acceptor_.async_accept(socket_,
    [this](boost::system::error_code ec)

Answer 1

It is a lambda expression used to create a function as an expression

[] is the capture list

A list of symbols can be passed as follows:

• [a,&b] where a is captured by value and b is captured by reference.
• [this] captures the this pointer by value
• [&] captures all automatic variables mentioned in the body of the lambda by reference
• [=] captures all automatic variables mentioned in the body of the lambda by value
• [] captures nothing

Answer 2

It is a part of a lambda expression. Look here for more info.