Lambda function as a default argument for std::function in constructor

c++, c++11, default-arguments, lambda

Solution

This is a bug: the compiler crashed or a fatal internal error occurred while processing the source code, and the message itself is kindly inviting you to treat the error as such:

"Please submit a full bug report, with preprocessed source if appropriate."

Here is another possible workaround:

template<class T>
class Filter
{
public:
    typedef std::function<bool(const T&)> FilterFunc;

    Filter() { }
    Filter(FilterFunc const& f) : f(f) { }

private:
    FilterFunc f = [](const T&){ return true; };
};

As a further alternative, GCC supports delegating constructors, which you may want to consider:

#include <functional>

template<class T>
class Filter
{
public:
    typedef std::function<bool(const T&)> FilterFunc;

    Filter() : Filter([](const T&){ return true; }) { }
    Filter(FilterFunc const& f) : f(f) { }

private:
    FilterFunc f;
};

Problem

I'd like to have a default functor for a functor parameter in the constructor of a class. As a minimal example I came up with a class which should server as a filter, which filters elements of type `T` iif a filter function returns true. The filter function should be provided in the constructor, defaulting to an "accept all" filter function: ``` template<class T> class Filter { public: typedef std::function<bool(const T&)> FilterFunc; Filter(const FilterFunc & f = [](const T&){ return true; }) : f(f) { } private: FilterFunc f; }; ``` I instantiate the template class like the following: ``` int main() { Filter<int> someInstance; // No filter function provided (<-- line 19) } ``` However, gcc 4.7 doesn't seem to like this piece of code: ``` prog.cpp: In constructor ‘Filter<T>::Filter(const FilterFunc&) [with T = int; Filter<T>::FilterFunc = std::function<bool(const int&)>]’: prog.cpp:19:17: internal compiler error: in tsubst_copy, at cp/pt.c:12141 Please submit a full bug report, with preprocessed source if appropriate. See <file:///usr/share/doc/gcc-4.7/README.Bugs> for instructions. Preprocessed source stored into /home/g9i3n9/cc82xcqE.out file, please attach this to your bugreport. ``` What's wrong? Is my code standard conformant (so GCC is really buggy here or hasn't implemented this) or am I doing something wrong? As a workaround, I currently use a default-constructed `std::function` and only call it (where I want to call it) if it was set: ``` Filter(const FilterFunc & f = FilterFunc) : f(f) { } // When using it: void process() { if (!f || f(someItem)) { // <-- workaround } } ```

Original source