How to create an array of anonymous functions in C++?

Question

C++ has 'lambdas' or anonymous functions. If they do not capture, they can be used in the place of function pointers. I could also declare an array of function pointers as follows:

double (*const Trig[])(double) = {sin, cos, tan};
cout << Trig[0](M_PI/2) << endl; // Prints 1

However I cannot figure out the correct syntax to use C++ lamdas in the place of function names in a global array initializer:

#include <iostream>
using namespace std;
static int (*const Func[])(int, int) = {
    [](int x, int y) -> int {
//   ^ error: expected expression
        return x+y;
    },
    [](int x, int y) -> int {
        return x-y;
    },
    [](int x, int y) -> int {
        return x*y;
    },
    [](int x, int y) -> int {
        return x/y;
    }
};
int main(void) {
    cout << Func[1](4, 6) << endl;
}

What is the correct way to initialize an array of pointers to anonymous functions in C++?

The code is OK. Upgrade your compiler.

The result of running g++ --version:

Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.0 (clang-1200.0.32.29)
Target: x86_64-apple-darwin19.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Compiler error from using g++ /Users/user/Lambda.cpp:

/Users/user/Lambda.cpp:4:3: error: expected expression
        [](int x, int y) -> int {
         ^
1 error generated.

How can I configure my compiler to accept this code?

Cannot reproduce your problem. Compiles fine for me, on gcc 12, and I get the expected result, -2. — Sam Varshavchik
– Sam Varshavchik, Commented Sep 29, 2022 at 19:57
it's interesting, though, that you invoke g++ and it seems to call clang. — Michael Kopp
– Michael Kopp, Commented Sep 29, 2022 at 20:05
@n.1.8e9-where's-my-sharem. I notice MSVC and gcc like it but clang does not. — Spencer
– Spencer, Commented Sep 29, 2022 at 20:05
Apple Clang also defaults to C++98 if I am not mistaken (as only of the current major compilers as far as I am aware), which is probably why the compiler is complaining about the lambda syntax in general. (Lambdas were introduced with C++11.) — user17732522
– user17732522, Commented Sep 29, 2022 at 20:20

CPlus · Accepted Answer · 2022-10-01 15:21:13Z

2

The code is correct. The problem was due to the compiler defaulting to an older version of C++, before support for lambda expressions was added in C++11.

All I had to do was to tell the compiler to use C++11 (or newer):

g++ /Users/user/Lambda.cpp --std=c++11

edited Oct 1, 2022 at 15:21

answered Sep 29, 2022 at 20:16

CPlus

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