为什么这段代码用g ++而不是MSVC ++编译？

Question

I'm trying to compile some open-source code ( https://github.com/BieremaBoyzProgramming/bbpPairings ), which I can get to compile on linux using g++ (v6.3.0), but which fails to compile in Visual Studio (VS Community 2019/16.1.5), with the somewhat obscure (to me, but my C++ is admittedly weak) error: "error C2143: syntax error: missing ';' before '<'".

The offending code in the source is here , but a minimal example extracted from the code is:

#include <iostream>
#include <random>

class Configuration {};

class MatchesConfiguration {
public:
    template <class RandomEngine>
    MatchesConfiguration(
        Configuration&&,
        RandomEngine&);
};

template <class RandomEngine>
MatchesConfiguration::MatchesConfiguration(
    Configuration&& configuration,
    RandomEngine& randomEngine) {}

template
MatchesConfiguration::MatchesConfiguration<std::minstd_rand>( // <--- SYNTAX ERROR HERE
    Configuration&&,
    std::minstd_rand&);

int main()
{
    std::cout << "Hello World!\n"; 
}

I've had a look at the MSDN description of the error code , but my grasp of C++ and templates is too tenuous to figure out what's going wrong. The project README says that C++14 is expected (with some optional C++17 stuff for FS stuff which shouldn't matter here I think), but as far as I can make out from the feature compatibility chart all of C++14 should be supported by VS 2019.

Answer 1

When you provide an explicit instantiation definition of a constructor (which does not really have a name according to the standard), you should do it by supplying the signature you'd like to instantiate, like so:

template
MatchesConfiguration::MatchesConfiguration(  // no <std::minstd_rand> here
    Configuration&&,
    std::minstd_rand&);

[ temp.arg.explicit#2 ]

Template arguments shall not be specified when referring to a specialization of a constructor template

---

Trivia from an old note (from 2006):
http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#581

" Can a templated constructor be explicitly instantiated or specialized? "

it is not possible to specify a constructor's template arguments in a constructor invocation (because the constructor has no name but is invoked by use of the constructor's class's name)

[...]

It was observed that explicitly specifying the template arguments in a constructor declaration is never actually necessary because the arguments are, by definition, all deducible and can thus be omitted.

Note that normal function templates can have non-deducible template parameters that must be supplied explicitly for an instantiation or specialization.

_{Thanks to Davis Herring and MM for guidance}