C ++宏和Lambda捕获

Question

When I use a lambda in a macro with multiple captures, I encounter one of these errors (Visual Studio 2017) :
Error C2143 syntax error: missing ']' before ';'
Error C2958 the left bracket '['

How can I avoid this error?

Code example :

#include <functional>
#include <iostream>

#define MYMACRO(lambda) lambda

int main()
{
    int a = 13;
    int b = 37;
    auto lambda = MYMACRO([a, b]() { std::cout << a << b << std::endl; });
    lambda();
    return 0;
}

Answer 1

GCC gives a slightly more friendly error message:

10:73: error: macro "MYMACRO" passed 2 arguments, but takes just 1

The commas in your lambda declaration are being interpreted as delimiting arguments to the macro. You need to wrap the expression in brackets:

#include <functional>
#include <iostream>

#define MYMACRO(lambda) lambda

int main()
{
    int a = 13;
    int b = 37;
    auto lambda = MYMACRO(([a, b]() { std::cout << a << b << std::endl; }));
    lambda();
    return 0;
}

Visual studio raises a warning then ignores spurious macro arguments so your code is equivalent to:

auto lambda = MYMACRO([a);

Which makes the error message more understandable. See https://docs.microsoft.com/en-gb/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4002

Answer 2

As noted elsewhere, the actual bug is that MYMACRO as written only expects a single argument. Unless enclosed by parentheses or quotes, commas are used to delineate separate arguments to a macro invocation.

A lambda may have multiple commas in the capture brackets, so MYMACRO actually needs to be able to handle variable arguments if you want to leave the invocation syntax the same as you have in your program. This is possible since C++11, which added support for variadic macros.

Visual Studio 2017 supports the variadic macro syntax. So, you could change your macro to be:

#define MYMACRO(...) __VA_ARGS__

Note that the variable arguments can only appear at the end of the macro arguments specification.