CODEWITHSUNDEEP
c++templatesvisual-c++lambdavariadic-templates
Michael Gunter
Michael Gunter

Reputation: 12811

Lambdas in variadic templates

Using Microsoft Visual C++ 2013 (12.0), I am encountering compile-time errors when using a lambda in a constructor in a variadic template. I have managed to boil it down as shown below (see the lines with the error comments). It appears to be a bug in 12.0 that is not present in 14.0. I haven't tried other versions. Is there any documentation on this bug, perhaps in the form of a release note that clarifies the conditions under which this bug occurs and which states that it has been explicitly fixed?

#include <functional>

// a simple method that can take a lambda
void MyFunction(const std::function<void()>& f) {}

// a simple class that can take a lambda
class MyClass
{
public:
    MyClass(const std::function<void()>& f) {}
};

// non-templated test
void test1()
{
    MyFunction([] {}); // OK
    MyClass([] {}); // OK
    MyClass o([] {}); // OK
}

// non-variadic template test
template<typename T>
void test2()
{
    MyFunction([] {}); // OK
    MyClass([] {}); // OK
    MyClass o([] {}); // OK
}

// variadic template test
template<typename... T>
void test3()
{
    MyFunction([] {}); // OK
    MyClass([] {}); // OK
    MyClass a([] {}); // error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
                      // error C2440: 'initializing' : cannot convert from 'test3::<lambda_12595f14a5437138aca1906ad0f32cb0>' to 'int'

    MyClass b(([] {})); // putting the lambda in an extra () seems to fix the problem
}

// a function using the templates above must be present
int main()
{
    test1();
    test2<int>();
    test3<int, int, int>();
    return 1;
}

Edit/Update: MSVC 2013 compiler seem to have this bug, latest versions fixed that. GCC and clang compilers don't show any error.

Upvotes: 28

Views: 1296

Answers (2)

Liastre
Liastre

Reputation: 1323

I can't say is that bug or not, back days Microsoft was not so eager to latest C++ standards, it was two steps behind GCC and CLang, and it's not a surprise to know that no-one implement a whole standard in one compiler update. Some stuff from std library may be partially implemented, for example Visual Studio usually implements new features under std::experimental namespace (filesystem for VS2013), but something like compiler lexemes can't be added that way.

For common information about standard compatibility I suggest to use compiler support article https://en.cppreference.com/w/cpp/compiler_support/11 (MSVC means _MSC_VER definition) and https://learn.microsoft.com/en-us/cpp/overview/visual-cpp-language-conformance more for detailed information about Visual Studio. Here https://en.wikipedia.org/wiki/Microsoft_Visual_C++#Internal_version_numbering you can find _MSC_VER version along with related Visual Studio version, source is https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros.

Based on those articles I believe that your problem lays in between "Variadic templates" and "Lambda expression" both are fully supported from Visual Studio 19.0, searching _MSC_VER 1900 in wiki that means Visual Studio 2015 (Update 2, if to be precise according to Microsoft article).

Upvotes: 1

henry3695
henry3695

Reputation: 29

vs2015 can be compiled normally, because vs2015 can fully support the c++11 standard, you can consider upgrading your vs

Upvotes: 0

Related Questions