c++11 lambda function with variable parameter list

Question

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Can a C++11 lambda function have a variable parameter list?

auto displayMessage = [objectLog] (const char * sFormat, ...) {
     ...
}

Refer to this: https://stackoverflow.com/questions/7627098/what-is-a-lambda-expression-in-c11 — Overflowh, Mar 18 '15 at 13:43
@Overflowh, that doesn't seem to say anything about using `...` in a C++11 lambda — Jonathan Wakely, Mar 18 '15 at 13:53

Jonathan Wakely · Answer 1 · 2015-03-18T14:03:58.903

Yes, the grammar allows it, and nothing forbids it.

A lambda-declarator is defined in 5.1.2 [expr.prim.lambda] as:

lambda-declarator:
( parameter-declaration-clause ) mutable_opt exception-specification_opt attribute-specifier-seq_opt trailing-return-type_opt

And a parameter-declaration-clause is defined in 8.3.5 [dcl.fct] as:

parameter-declaration-clause:
parameter-declaration-list_opt ..._opt
parameter-declaration-list , ...

Also, G++, Clang and EDG all accept it without problems.

Under the hood, a C++11 lambda generates closure type with an operator() member function, and there is no reason that function can't have ..., e.g. this lambda expression:

auto l = [](int, ...) { }

generates a type like this:

struct unnamed_closure_type {
  void operator()(int, ...) const { }
};

which is perfectly valid C++.

In fact, Clang won't allow you to use `va_start` in such lambda, it's a known bug (https://llvm.org/bugs/show_bug.cgi?id=19617) — Anton Savin, Mar 18 '15 at 13:55

Peter VARGA · Accepted Answer · 2015-03-18T16:18:15.533

It was easier than I thought and as meant before nothing forbids it.

I must admit I could have tried it before but I am a C++ beginner and therefore I thought it is more complicated. Here is a code for a lambda function with variable parameter list. This lambda function is used for internal logging - either directly to stdout or to a file.

auto displayMessage = [objectLog] (const char * sFormatString, ...)
{
    char sMessage [MAX_LOG_LINE_LENGTH];
    va_list vArgPtr;
    va_start(vArgPtr, sFormatString);

    vsnprintf(sMessage, sizeof(sMessage), sFormatString, vArgPtr);
    // EDIT: according to Jonathan Wakely's comment - added va_end()...
    va_end(vArgPtr);

    if ( objectLog )
    {
        objectLog->insertLogEntry("%s", sMessage);
    }
    else
    {
        printf("%s", sMessage);
    }
};

You're missing `va_end(vArgPtr)` – Jonathan Wakely Mar 18 '15 at 16:06 — Jonathan Wakely, Mar 18 '15 at 16:06

score 0 · Answer 3 · answered Mar 18 '15 at 13:45

0

I just tried with Apple LLVM 6 (LLVM 3.5svn) and it compiled just fine. I guess there's nothing very different from regular function calls here, with the only forbidden thing being the use of the auto keyword.

answered Mar 18 '15 at 13:45

sansuiso

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c++11 lambda function with variable parameter list

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