What does this oddly syntaxed C++ code with mean?

Question

I came across this code and I can not make sense of it.

RegisterCallback(MgrTsk::NAME,                                                                                                               
    [=](uint16_t cmd, uint16_t value, uint32_t size, void* pData) -> bool     {                                                              
        return MsgFromTsk(cmd, size, pData);                                                                                             
});

The return type of MsgFromTsk is bool. The API for RegisterCallback is -

template<typename F>                                                                                                                          
void RegisterCallback(const char* procName, F msgCallback)

This probably might be a simple question, but even after a lot of google-ing I couldn't understand the syntax.

Which part of this confuses you? Do you know the lambda syntax? — UnholySheep, Jul 06 '18 at 11:51
https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11 — James Poag, Jul 06 '18 at 11:55
It's a *lambda* function with *trailing return type* syntax. — Bathsheba, Jul 06 '18 at 11:57
@UnholySheep I was unaware on the lambda syntax, thanks for all your help — tinkerbeast, Jul 06 '18 at 12:01

score 1 · Answer 1 · edited Jul 06 '18 at 12:02

In the call to RegisterCallback in the first block you posted, that function is called with MgrTsk::NAME as a first argument and

 [=](uint16_t cmd, uint16_t value, uint32_t size, void* pData) -> bool {                                                              
    return MsgFromTsk(cmd, size, pData);}

as a second argument.

Thas is, the template parameter F is now a lambda that takes uint16_t, uint16_t, uint32_t, void* as arguments and returns bool, and it calls MsgFromTsk to determine the returned value.

score 1 · Accepted Answer · answered Jul 06 '18 at 11:59

This is a lambda:

[=](uint16_t cmd, uint16_t value, uint32_t size, void* pData) -> bool     {                                                              
    return MsgFromTsk(cmd, size, pData);                                                                                             
}

[...] is the capture-list of the lambda,
The =in [=] means that by default, the lambda captures variables by value,
(...) are the parameters passed to the lambda,
-> bool is the return type of the lambda,
{...}is the body of the lambda

What does this oddly syntaxed C++ code with mean?

2 Answers2