C++ 对类型值的引用绑定删除限定符

binding of reference to a value of type drops qualifiers

提问by dcrearer

I have the following source from the Text Accelerated C++. When I attempt to compile the source file I get the following compilation error listed below. I'm a novice of the C++ language so your assistance would be appreciated.

我有来自文本加速 C++ 的以下来源。当我尝试编译源文件时，出现下面列出的编译错误。我是 C++ 语言的新手，因此不胜感激。

#include <algorithm>
#include <iomanip>
#include <ios>
#include <iostream>
#include <string>
#include <vector>
#include <stdexcept>

using namespace std;

struct Student_info {
    string name;
    double midterm, final;
    vector<double> homework;
};

double median(vector<double>);
double grade(const Student_info&);
double grade(double, double, double);
double grade(double, double, const vector<double>&);
istream& read_hw(istream&, vector<double>&);
istream& read(istream&, Student_info&);
bool compare(const Student_info&, Student_info&);

int main() {

    vector<Student_info> students;
    Student_info record;
    string::size_type maxlen = 0;

    while(read(cin, record)) {
        maxlen = max(maxlen, record.name.size());
        students.push_back(record);
    }

    sort(students.begin(), students.end(), compare);

    for(vector<Student_info>::size_type i = 0;
            i != students.size(); ++i) {

        cout << students[i].name << string(maxlen + 1 - students[i].name.size(), ' ');

        try {
            double final_grade = grade(students[i]);
            streamsize prec = cout.precision();
            cout << setprecision(3) << final_grade
                    << setprecision(prec);
        } catch(domain_error& e) {
            cout << e.what();
        }
        cout << endl;
    }
    return 0;
}

double median(vector<double> vec) {
    typedef vector<double>::size_type vec_sz;
    vec_sz size = vec.size();

    if(size == 0)
        throw domain_error("median of an empty vector");

    sort(vec.begin(), vec.end());
    vec_sz mid = size/2;

    return size%2 == 0 ? (vec[mid] + vec[mid - 1])/2 : vec[mid];
}
double grade(const Student_info& s) {
    return grade(s.midterm, s.final, s.homework);
}
double grade(double midterm, double final, double homework) {
    return 0.2*midterm + 0.4*final + 0.4*homework;
}
double grade(double midterm, double final, const vector<double>& hw) {
    if(hw.size() == 0)
        throw domain_error("student has done no homework");
    return grade(midterm, final, median(hw));
}
istream& read_hw(istream& in, vector<double>& hw) {
    if(in) {
        hw.clear();

        double x;
        while(in >> x)
            hw.push_back(x);
        in.clear();
    }

    return in;
}
istream& read(istream& is, Student_info& s) {
    is >> s.name >> s.midterm >> s.final;
    read_hw(is, s.homework);
    return is;
}
bool compare(const Student_info& x, const Student_info& y) {
    return x.name < y.name;
}

binding of reference to type 'Student_info' to a value of type 'const Student_info' drops qualifiers compute_grades_rev-b
line 125, external location: /usr/include/c++/4.2.1/bits/stl_algo.h C/C++ Problem

将“Student_info”类型的引用绑定到“const Student_info”类型的值删除限定符compute_grades_rev-b
第125行，外部位置：/ usr/include /c++/ 4.2.1/bits/stl_algo.h C/C++问题

Here is the code from stl_algo.h:

这是来自 stl_algo.h 的代码：

 template<typename _Tp, typename _Compare>
    inline const _Tp&
    __median(const _Tp& __a, const _Tp& __b, const _Tp& __c, _Compare __comp)
    {
      // concept requirements
      __glibcxx_function_requires(_BinaryFunctionConcept<_Compare,bool,_Tp,_Tp>)
      if (__comp(__a, __b))
    if (__comp(__b, __c))
      return __b;
    else if (__comp(__a, __c))
      return __c;
    else
      return __a;
      else if (__comp(__a, __c))
    return __a;
      else if (__comp(__b, __c))
    return __c;
      else
    return __b;
    }

I change the declaration of the compare function from:

我将比较函数的声明从：

bool compare(const Student_info&, Student_info&);
bool compare(const Student_info, Student_info);

Now it compiles.

现在它编译了。

回答by David Rodríguez - dribeas

The error is indicating that you cannot bind a non-const reference to a const-object, as that would drop(discardin other compiler's errors), disregard or ignore the constqualifier.

该错误表明您不能将非常量引用绑定到常量对象，因为这会丢弃（在其他编译器的错误中丢弃）、忽略或忽略const限定符。

What it tries to indicate is that if the operation was allowed you would be able to modify the object through the reference ignoring the fact that the object itself is const, breaking const-correctness.

它试图表明的是，如果允许操作，您将能够通过引用修改对象，而忽略对象本身是 的事实，从而const破坏了常量正确性。

In your particular code, the function __medianin the library takes __a, __b, and __cby const reference and tries to call the __compfunction, which in your program (first declaration) takes the second argument by non-const reference. To be able to call __comp(__a,__b)(or any other call to __compin that function) it would have to bind an object accessible only through a const&to the second argument that takes a non-const reference. This is most probably a typo, since you define comparebelow with both arguments being const references.

在您的特定代码中，__median库中的函数通过 const 引用接受__a、__b和__c并尝试调用该__comp函数，该函数在您的程序（第一个声明）中通过非常量引用接受第二个参数。为了能够调用__comp(__a,__b)（或__comp该函数中的任何其他调用），它必须将只能通过 a 访问的对象绑定const&到采用非常量引用的第二个参数。这很可能是一个错字，因为您compare在下面定义了两个参数都是 const 引用。

Change the declaration of comparebefore main to:

将comparebefore main的声明改为：

bool compare(const Student_info&, const Student_info&);
//                                ^^^^^