c++ - 奇怪的类型名用法 c++11

Question

我有以下代码

#include <iostream>
#include <string>
using namespace std;

template<class T> struct Tpl;
template<>        struct Tpl<int>    { void print() { cout << "int"    << endl; } };
template<>        struct Tpl<string> { void print() { cout << "string" << endl; } };

int main() {
  typename Tpl<int>::Tpl{}.print();
  typename Tpl<int>::Tpl<string>{}.print();
  typename Tpl<int>::Tpl<int>{}.print();
  typename Tpl<int>::Tpl::Tpl{}.print();
  typename Tpl<string>::Tpl<int>{}.print();
  typename Tpl<int>::Tpl::Tpl<int>{}.print();
  typename Tpl<string>::Tpl::Tpl<int>::Tpl{}.print();
  typename Tpl<int>::Tpl<string>::Tpl<int>::Tpl<string>::Tpl<int>::Tpl<string>{}.print();
}

给出以下输出

int
string
int
int
int
int
int
string

我的问题是 typename 在这些情况下如何工作，特别是在最后一个您可以在不同模板参数之间切换的情况下。我想知道这种行为是标准的还是编译器有些奇怪。

这是编译器的信息:

> g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.8.4-2ubuntu1~14.04' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs --enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.8 --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object --disable-libmudflap --enable-plugin --with-system-zlib --disable-browser-plugin --enable-java-awt=gtk --enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre --enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64 --with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64 --with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar --enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.4 (Ubuntu 4.8.4-2ubuntu1~14.04)

Answer 1

这是因为 [temp.local]/1:

Like normal (non-template) classes, class templates have an injected-class-name (Clause 9). The injected-class-name can be used as a template-name or a type-name.