在使用GCC 4.7.2和Clang 3.1编译一些C++11代码时,我遇到了一个问题,Clang无法推断出模板参数,而GCC可以成功地推断出来。
更抽象地说,代码看起来像这样:
src/test.cc:
struct Element {
};
template <typename T>
struct FirstContainer {
};
template <typename T, typename U = Element>
struct SecondContainer {
};
template <template <typename> class Container>
void processOrdinary(Container<Element> /*elements*/) {
}
template <template <typename, typename> class Container>
void processOrdinary(Container<Element, Element> /*elements*/) {
}
template <template <typename, typename...> class Container>
void processVariadic(Container<Element> /*elements*/) {
}
int main() {
// This function instantiation works in both GCC and Clang.
processOrdinary(FirstContainer<Element>{});
// This function instantiation works in both GCC and Clang.
processOrdinary(SecondContainer<Element>{});
// This function instantiation works in both GCC and Clang.
processVariadic(FirstContainer<Element>{});
// This function instantiation works in both GCC and Clang.
processVariadic<SecondContainer>(SecondContainer<Element>{});
// This function instantiation works in GCC but not in Clang.
processVariadic(SecondContainer<Element>{});
return 0;
}
通过阅读标准中§14.3.3的示例和§14.8.2的规范,我认为推断应该是可行的,但我不能确定。以下是构建时得到的输出:
mkdir -p build-gcc/
g++ -std=c++0x -W -Wall -Wextra -Weffc++ -pedantic -c -o build-gcc/test.o src/test.cc
g++ -o build-gcc/test build-gcc/test.o
mkdir -p build-clang/
clang++ -std=c++11 -Weverything -Wno-c++98-compat -c -o build-clang/test.o src/test.cc
src/test.cc:34:3: error: no matching function for call to 'processVariadic'
processVariadic(SecondContainer<Element>{});
^~~~~~~~~~~~~~~
src/test.cc:21:6: note: candidate template ignored: failed template argument deduction
void processVariadic(Container<Element> /*elements*/) {
^
1 error generated.
make: *** [build-clang/test.o] Fel 1
结果为什么会不同?是GCC粗心、Clang愚蠢,还是我的代码包含了未指定的行为或者以上情况都有可能导致结果差异。
typedef int Element;
,对吗? - Quuxplusone