A Workaround for Lambda ODR Violations

As brought up in the post https://www.reddit.com/r/cpp/comments/40lm8o/lambdas_are_dangerous/ with lambdas in inline functions you can run into ODR violations and thus undefined behavior.

There is also a stack overflow discussion at http://stackoverflow.com/questions/34717823/does-using-lambda-in-header-file-violate-odr

While, the ultimate fix may rely with the Core Working Group, I think here is a work around.

The basis for the trick come from Paul Fultz II in a post about constexpr lambda. You can find the post at http://pfultz2.com/blog/2014/09/02/static-lambda/

Here is some problematic code from the stackoverflow discussion. The lambda may have a different type across translation units and thus result in different specializations of for_each being called for different translation units resulting in ODR violations and thus undefined behavior.
 

inline void g() {
    int arr[2] = {};
    std::for_each(arr, arr+2, [] (int i) {std::cout << i << ' ';});
}

Here is a simple fix that will prevent the ODR violation.

// Based on Richard Smith trick for constexpr lambda
// via Paul Fultz II (http://pfultz2.com/blog/2014/09/02/static-lambda/)
template<typename T>
auto addr(T &&t)
{
    return &t;
}

static const constexpr auto odr_helper = true ? nullptr : addr([](){});

template <class T = decltype(odr_helper)>
inline void g() {
    int arr[2] = {};
    std::for_each(arr, arr+2, [] (int i) {std::cout << i << ' ';});
}

We create a static const constexpr null pointer with the type of a lambda. If lambdas are different types across different translation units then odr_helper will have different types across different translation units. Because g now is a template function using the type of odr_helper, g will be a different specialization across different translation units and thus will not result in an odr violation.

Also note that because T is defaulted, g can be used without any changes from before.

ideone at https://ideone.com/NdBpXN