没有匹配函数调用类型为“hash<char">”的对象（Stroustrup Book Drill）

Question

我开始使用《编程：使用 C++ 的实践和原则》一书学习 C++

在我的第一次练习中，它给了我一些代码来运行，并说我应该得到一个错误，我应该向其他人寻求帮助。这就是我正在做的事情。

错误（如Xcode所示，在头文件中）：

No matching function for call to object of type 'hash<char">' 

这是 .cpp 文件

#include "std_lib_facilities.h"

int main ()
{
    cout << "Hello, World!\n"; //output "Hello, World!"
    keep_window_open(); //wait for a character to be entered
    return 0;

}

头文件： （在 Xcode 中，分歧在于 return hash<char*>()(s.c_str()); 行，它在第 43 行附近。

/*
    simple "Programming: Principles and Practice using C++" course header to
    be used for the first few weeks.
    It provides the most common standard headers (in the global namespace)
    and minimal exception/error support.

    Students: please don't try to understand the details of headers just yet.
    All will be explained. This header is primarily used so that you don't have
    to understand every concept all at once.

    Revised April 25, 2010: simple_error() added
*/

#ifndef H112
#define H112 201004L

#include<iostream>
#include<fstream>
#include<sstream>
#include<cmath>
#include<cstdlib>
#include<string>
#include<list>
#include<vector>
#include<algorithm>
#include<stdexcept>

//------------------------------------------------------------------------------

#ifdef _MSC_VER
#include <hash_map>
using stdext::hash_map;
#else
#include <ext/hash_map>
using __gnu_cxx::hash_map;

namespace __gnu_cxx {

    template<> struct hash<std::string>
    {
        size_t operator()(const std::string& s) const
        {
            return hash<char*>()(s.c_str()); //HERE IT IS!!!!!!!!
        }
    };

} // of namespace __gnu_cxx
#endif

//------------------------------------------------------------------------------

#define unordered_map hash_map

//------------------------------------------------------------------------------

typedef long Unicode;

//------------------------------------------------------------------------------

using namespace std;

template<class T> string to_string(const T& t)
{
    ostringstream os;
    os << t;
    return os.str();
}

struct Range_error : out_of_range { // enhanced vector range error reporting
    int index;
    Range_error(int i) :out_of_range("Range error: "+to_string(i)), index(i) { }
};


// trivially range-checked vector (no iterator checking):
template< class T> struct Vector : public std::vector<T> {
    typedef typename std::vector<T>::size_type size_type;

    Vector() { }
    explicit Vector(size_type n) :std::vector<T>(n) {}
    Vector(size_type n, const T& v) :std::vector<T>(n,v) {}
    template <class I>
    Vector(I first, I last) :std::vector<T>(first,last) {}

    T& operator[](unsigned int i) // rather than return at(i);
    {
        if (i<0||this->size()<=i) throw Range_error(i);
        return std::vector<T>::operator[](i);
    }
    const T& operator[](unsigned int i) const
    {
        if (i<0||this->size()<=i) throw Range_error(i);
        return std::vector<T>::operator[](i);
    }
};

// disgusting macro hack to get a range checked vector:
#define vector Vector

// trivially range-checked string (no iterator checking):
struct String : std::string {

    String() { }
    String(const char* p) :std::string(p) {}
    String(const string& s) :std::string(s) {}
    template<class S> String(S s) :std::string(s) {}
    String(int sz, char val) :std::string(sz,val) {}
    template<class Iter> String(Iter p1, Iter p2) : std::string(p1,p2) { }

    char& operator[](unsigned int i) // rather than return at(i);
    {
        if (i<0||size()<=i) throw Range_error(i);
        return std::string::operator[](i);
    }

    const char& operator[](unsigned int i) const
    {
        if (i<0||size()<=i) throw Range_error(i);
        return std::string::operator[](i);
    }
};

#ifndef _MSC_VER
namespace __gnu_cxx {

    template<> struct hash<String>
    {
        size_t operator()(const String& s) const
        {
            return hash<std::string>()(s);
        }
    };

} // of namespace __gnu_cxx
#endif


struct Exit : runtime_error {
    Exit(): runtime_error("Exit") {}
};

// error() simply disguises throws:
inline void error(const string& s)
{
    throw runtime_error(s);
}

inline void error(const string& s, const string& s2)
{
    error(s+s2);
}

inline void error(const string& s, int i)
{
    ostringstream os;
    os << s <<": " << i;
    error(os.str());
}

#if _MSC_VER<1500
    // disgusting macro hack to get a range checked string:
    #define string String
    // MS C++ 9.0 have a built-in assert for string range check
    // and uses "std::string" in several places so that macro substitution fails
#endif

template<class T> char* as_bytes(T& i)  // needed for binary I/O
{
    void* addr = &i;    // get the address of the first byte
                        // of memory used to store the object
    return static_cast<char*>(addr); // treat that memory as bytes
}


inline void keep_window_open()
{
    cin.clear();
    cout << "Please enter a character to exit\n";
    char ch;
    cin >> ch;
    return;
}

inline void keep_window_open(string s)
{
    if (s=="") return;
    cin.clear();
    cin.ignore(120,'\n');
    for (;;) {
        cout << "Please enter " << s << " to exit\n";
        string ss;
        while (cin >> ss && ss!=s)
            cout << "Please enter " << s << " to exit\n";
        return;
    }
}



// error function to be used (only) until error() is introduced in Chapter 5:
inline void simple_error(string s)  // write ``error: s�� and exit program
{
    cerr << "error: " << s << '\n';
    keep_window_open();     // for some Windows environments
    exit(1);
}

// make std::min() and std::max() accessible:
#undef min
#undef max

#include<iomanip>
inline ios_base& general(ios_base& b)   // to augment fixed and scientific
{
    b.setf(ios_base::fmtflags(0),ios_base::floatfield);
    return b;
}

// run-time checked narrowing cast (type conversion):
template<class R, class A> R narrow_cast(const A& a)
{
    R r = R(a);
    if (A(r)!=a) error(string("info loss"));
    return r;
}


inline int randint(int max) { return rand()%max; }

inline int randint(int min, int max) { return randint(max-min)+min; }

inline double sqrt(int x) { return sqrt(double(x)); }   // to match C++0x

#endif

无意侵犯版权

Answer 1

这是一个常量问题。 Clang 报告：候选函数不可行：第一个参数 ('const value_type *' (aka 'const char *')) 将丢失 const 限定符。

试试：

return hash<const char*>()(s.c_str()); 

以前你有：

return hash<char *>()(s.c_str());

但是 s.c_str() 返回一个 const char * 并且您不能将 const char 传递给期望 char * 的函数。请参阅C++ Const Usage Explanation 了解一些血腥细节。

另外：顶级提示。在 Xcode 中出现错误时。按 Command-4。打开错误旁边的小下拉箭头以查看更多信息，还有有用的、令人惊奇的、拯救生命的、节省咖啡的附加信息:)