CODEWITHSUNDEEP
c++templatesc++11lambdadefault-parameters
Osuvaldo
Osuvaldo

Reputation: 274

Lambda as default parameter to a member function template

Header File

#include <functional>

struct Check {};
struct Entry {};
struct Table {};

class SeatEntries
{
public:
    // does not compile!
    template<class T>
    void Populate(T from,
                  std::function<bool (Entry *)> predicate = [] (Entry *) { return true; },
                  std::function<void (Entry *)> action = [] (Entry *) {})
    {
        Reset();
        PopulateImpl(from, predicate, action);
    }

    // compiles!
    void PopulateFromCheck(Check *from,
                  std::function<bool (Entry *)> predicate = [] (Entry *) { return true; },
                  std::function<void (Entry *)> action = [] (Entry *) {})
    {
        Reset();
        //PopulateImpl(from, predicate, action);
    }

    void Reset();

private:
    template<class T>
    void PopulateImpl(T from,
                      std::function<bool (Entry *)> predicate,
                      std::function<void (Entry *)> action);
};

template<>
void SeatEntries::PopulateImpl<Table *>(Table *from, std::function<bool (Entry *)> predicate, std::function<void (Entry *)> action);

template<>
void SeatEntries::PopulateImpl<Check *>(Check *from, std::function<bool (Entry *)> predicate, std::function<void (Entry *)> action);

Question

Why doesn't the first member function template compile? Is this a bug in both VS2012 and gcc 4.8.1? Am I ignorant to some fact concerning member function templates and lambdas as default parameters?

VS2012 Output

error C2958: the left parenthesis '(' found at 'seathelpers.h(33)' was not matched correctly  
error C2988: unrecognizable template declaration/definition  
error C2059: syntax error : '{'  
error C2334: unexpected token(s) preceding '{'; skipping apparent function body

The left parentheses referenced is the opening parentheses to the Populate() function template.

GCC 4.8.1 (MinGW)

Test.h:13:66: internal compiler error: in push_class_level_binding_1, at cp/name-lookup.c:3019  
    std::function<bool (Entry *)> predicate = [] (Entry *) { return true; },  
                                               ^

Upvotes: 3

Views: 687

Answers (1)

Sam Varshavchik
Sam Varshavchik

Reputation: 118292

I was able to compile the above code, without errors, with gcc 4.8.3

$ g++ --version
g++ (GCC) 4.8.3 20140624 (Red Hat 4.8.3-1)

Upvotes: 1

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