Variadic模板問題

Question

我想寫一個通用代碼來比較使用其target（）模板方法的std :: functions。這裏是我的非通用示例代碼：

#include <cstdio> 
#include <functional> 

static void bar() {} 
static void baz() {} 

bool cmp(std::function<void()> f1, std::function<void()> f2) 
{ 
    void (**t1)() = f1.target<void(*)()>(); 
    void (**t2)() = f2.target<void(*)()>(); 
    return (!t1 && !t2) || (t1 && t2 && *t1 == *t2); 
} 

int main(int argc, char *argv[]) 
{ 
    std::function<void()> f1(bar), f2(baz), f3(bar); 
    printf("equal:  %d\n", cmp(f1, f3)); 
    printf("non-equal: %d\n", cmp(f1, f2)); 
    return 0; 
} 

這個編譯和運行良好，使用gcc 4.6.1 -std = C++ -x。然而，當我試圖編譯解析錯誤代碼以下通用CMP函數編譯失敗：

#include <cstdio> 
#include <functional> 

static void bar() {} 
static void baz() {} 


template<typename Result, typename ... Args> 
bool cmp(std::function<Result(Args...)> f1, std::function<Result(Args...)> f2) 
{ 
    Result (**t1)(Args...) = f1.target<Result(*)(Args...)>(); 
    Result (**t2)(Args...) = f2.target<Result(*)(Args...)>(); 
    return (!t1 && !t2) || (t1 && t2 && *t1 == *t2); 
} 

int main(int argc, char *argv[]) 
{ 
    std::function<void()> f1(bar), f2(baz), f3(bar); 
    printf("equal:  %d\n", cmp(f1, f3)); 
    printf("non-equal: %d\n", cmp(f1, f2)); 
    return 0; 
} 

錯誤代碼是：

functional.cpp: In function ‘bool cmp(std::function<_Res(_ArgTypes ...)>, std::function<_Res(_ArgTypes ...)>)’: 
functional.cpp:11:44: error: expected primary-expression before ‘(’ token 
functional.cpp:11:46: error: expected primary-expression before ‘)’ token 
functional.cpp:11:52: error: expected primary-expression before ‘...’ token 
functional.cpp:11:58: error: expected primary-expression before ‘)’ token 
functional.cpp:12:44: error: expected primary-expression before ‘(’ token 
functional.cpp:12:46: error: expected primary-expression before ‘)’ token 
functional.cpp:12:52: error: expected primary-expression before ‘...’ token 
functional.cpp:12:58: error: expected primary-expression before ‘)’ token 

任何提示？

Answer 1

這應該可能會被

f1.template target<Result(*)(Args...)>() 
    ^^^^^^^^ 

和下一行類似。