Syntax help: Function pointer surrounded by square brackets

Question

I'm confused about the use of square brackets around [funcParam] in the C++11 code segment below:

typedef std::function<std::vector<OtherType> ()> FuncPtr;

inline void create(FuncPtr funcParam)
{
    auto create_evaluator = [funcParam] ()
    {
        return anotherFunction(funcParam());
    };
    // ...
}

The code above it called by this line (somewhat simplified to keep it readable):

create( [] () { return CONSTANT; } );

Can someone explain the use of brackets in both situations? In the calling code, it looks to be used to create a function without a name. What's it doing in the first segment? Thanks!

This is called lambda-expression. Google "C++ 11 lambda" and you will find plenty of information. — Victor Havin, Mar 05 '18 at 21:53
Exactly what I needed, thank you! (Very hard to search for this without "lambda"). — Kayvon, Mar 05 '18 at 21:56

score 1 · Accepted Answer · answered Mar 05 '18 at 22:18

create_evaluator is a lambda expression (check out the examples at the bottom of that page).

In the case of

auto create_evaluator = [funcParam] ()
{
    return anotherFunction(funcParam());
};

[funcParam] is a variable in the local scope that is captured by the lambda function and therefore available to be reference within the lambda function when it is called elsewhere.
() denotes the arguments to the lambda function (none in this case)
{ return anotherFunction(funcParam()); } is the body of the lambda function.

With the call

create( [] () { return CONSTANT; } );

create is called with another lambda expression as its argument. That lambda argument

captures nothing: []
has no arguments: ()
has the body: { return CONSTANT; }

Syntax help: Function pointer surrounded by square brackets

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