c++ - 传递初始化列表时可变参数模板参数推导失败

Question

Bar 包含 std::array 的 std::pair 的 std::vector FooValueAdaptor 的。FooValueAdaptor 将 int 隐式转换为 bool 为 FooValue，这在这个人为的示例中没有什么意义，但在我的应用程序。我实现了一个方便的函数 Bar::addEntries 用于一次添加多个条目，但是使用两个以上的参数调用它无法使用 GCC 4.8.0 进行编译。请参阅下面的错误消息。

#include <array>
#include <utility>
#include <vector>

enum class FooValue {
    A,
    B,
    C
};

class FooValueAdaptor {
public:
    FooValueAdaptor(bool value)
        : m_value(static_cast<FooValue>(value)) {
    }

    FooValueAdaptor(int value)
        : m_value(static_cast<FooValue>(static_cast<bool>(value))) {
    }

    FooValueAdaptor(FooValue value)
        : m_value(value) {
    }

    operator FooValue() {
        return m_value;
    }

    operator bool() {
        return m_value == FooValue::C;
    }

private:
    FooValue m_value;
};

template<std::size_t nFirst, std::size_t nSecond>
class Bar {
public:
    typedef std::array<FooValueAdaptor, nFirst> First;
    typedef std::array<FooValueAdaptor, nSecond> Second;
    typedef std::pair<First, Second> Entry;

    Bar()
        : m_table() {
    }

    void addEntry(First first, Second second) {
        m_table.push_back(std::make_pair(first, second));
    }

    template <typename... Args>
    void addEntries() {
    }

    template <typename... Args>
    void addEntries(First first, Second second, Args... args) {
        addEntry(first, second);
        addEntries(args...);
    }

private:
    std::vector<Entry> m_table;
};

int main(int argc, char **argv) {
    Bar<2, 1> b;

    b.addEntry({ 0, 0 }, { 0 });
    b.addEntries(
        { 0, 1 }, { 0 },
        { 1, 0 }, { 0 },
        { 1, 1 }, { 1 }
    );

    return 0;
}

编译器错误信息:

test.cpp: In function ‘int main(int, char**)’:
test.cpp:74:2: error: no matching function for call to ‘Bar<2ul, 1ul>::addEntries(<brace-enclosed initializer list>, <brace-enclosed initializer list>, <brace-enclosed initializer list>, <brace-enclosed initializer list>, <brace-enclosed initializer list>, <brace-enclosed initializer list>)’
  );
  ^
test.cpp:74:2: note: candidates are:
test.cpp:53:7: note: template<class ... Args> void Bar<nFirst, nSecond>::addEntries() [with Args = {Args ...}; long unsigned int nFirst = 2ul; long unsigned int nSecond = 1ul]
  void addEntries() {
       ^
test.cpp:53:7: note:   template argument deduction/substitution failed:
test.cpp:74:2: note:   candidate expects 0 arguments, 6 provided
  );
  ^
test.cpp:57:7: note: void Bar<nFirst, nSecond>::addEntries(Bar<nFirst, nSecond>::First, Bar<nFirst, nSecond>::Second, Args ...) [with Args = {}; long unsigned int nFirst = 2ul; long unsigned int nSecond = 1ul; Bar<nFirst, nSecond>::First = std::array<FooValueAdaptor, 2ul>; Bar<nFirst, nSecond>::Second = std::array<FooValueAdaptor, 1ul>]
  void addEntries(First first, Second second, Args... args) {
       ^
test.cpp:57:7: note:   candidate expects 2 arguments, 6 provided

我怎样才能帮助编译器进行推导？

Answer 1

你需要明确地告诉编译器你需要什么:

void addEntries(std::initializer_list<std::pair<First, Second>> il) {
   for( const auto& e : il ) {
      addEntry(e.first,e.second);
   }
}

并这样调用它:

b.addEntry({{ 0, 0 }}, {{ 0 }});
b.addEntries({
    {{{ 0, 1 }}, {{ 0 }}},
    {{{ 1, 0 }}, {{ 0 }}},
    {{{ 1, 1 }}, {{ 1 }}}
});

请注意大量的大括号，但我认为以上实际上是唯一正确的语法。 GCC 4.8 和 Clang 3.2 都接受更少的括号，但 Clang 给出了很多警告，上面修复了这个问题。有些人已经working on a "fix" ，但这需要一些时间。