C++ lambdas, "error: expected expression"

phoenix Published at Dev

Phoenix

I'm trying to reproduce java's streams API in C++, and have made this program so far.

#include <iostream>

using namespace std;

template <typename E>
class stream {
    virtual void collect(void (*consumer) (E)) = 0;
    virtual bool anyMatch(bool (*predicate) (E)) {
        bool found = false;
        collect([&found](E obj) -> { if (predicate(obj)) {found = true} });
        return found;
    }
};

int main() {
    return 0;
}

But when I try to compile it with g++:

What am I doing wrong the lambda? It is supposed to provide a function (the consumer) which will test the given E with the predicate (a function), and if it yields true, set found to true.

Vittorio Romeo

There are a few separate issues in your code:

The error you posted suggests that you're compiling in pre-C++11 mode. Lambdas were introduced in C++11.
There are several syntax errors. Your lambda trailing return type is missing the type, and you're missing a semicolon in the body of the lambda.
You're trying to convert a non-captureless lambda to a function pointer. This is impossible, as capturing variables requires a state/context.

Your code is not valid C++ - the syntax is incorrect. Here's a version with valid syntax:

template <typename E>
class stream
{
    virtual void collect(void (*consumer)(E)) = 0;
    virtual bool anyMatch(bool (*predicate)(E))
    {
        bool found = false;
        collect([predicate, &found](E obj)
            {
                if(predicate(obj))                    
                    found = true;                    
            });

        return found;
    }
};

Nevertheless, the code will not compile because non-captureless lambdas cannot be converted to function pointers. If that was allowed, it would be a recipe for disaster, as the information regarding the captured variables will be lost. You can instead use std::function, which erases the type of the lambda and works with non-empty capture lists, at the cost of memory/run-time overhead:

template <typename E>
class stream
{
    virtual void collect(std::function<void(E)> consumer)
    {
        (void)consumer;
    }

    virtual bool anyMatch(std::function<bool(E)> predicate)
    {
        bool found = false;
        collect([predicate, &found](E obj)
            {
                if(predicate(obj))                    
                    found = true;                    
            });

        return found;
    }
};

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Please contact [email protected] to delete if infringement.

edited at2021-03-4

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