我正在读一本书(Accelerated C++)并从那本书中做练习,试图自己学习 C++。
现在我试图理解为什么以下代码不起作用:
// analysis.cpp
void write_analysis(ostream& out, const string& name,
double analysis(const vector<Student_info>&),
const vector<Student_info>& did,
const vector<Student_info>& didnt)
{
out << name << ":median(did) = " << analysis(did) <<
", median(didnt)=" << analysis(didnt) << endl;
}
template <double F(Student_info&)>
double generic_analysis(const vector<Student_info>& students)
{
vector<double> grades;
transform(students.begin(), students.end(),
back_inserter(grades), F);
return median(grades);
}
template<> double generic_analysis<grade_aux>(const vector<Student_info>& students);
double median_analysis(const vector<Student_info>& students)
{
return generic_analysis<grade_aux>(students);
// if not using template function, the code will be
// as following
// vector<double> grades;
// transform(students.begin(), students.end(),
// back_inserter(grades), grade_aux);
// return median(grades);
}
分析.cpp
// analysis.h
template <double F(const Student_info&)>
double generic_analysis(const std::vector<Student_info>&);
void write_analysis(std::ostream&, const std::string&,
double analysis(const std::vector<Student_info>&),
const std::vector<Student_info>&,
const std::vector<Student_info>&);
double grade_aux(const Student_info&);
double median_analysis(const std::vector<Student_info>&);
分析.h
// main.cpp
int main()
{
// students who did and didn't do all their homework
vector<Student_info> did, didnt;
// read the student records and partition them
Student_info student;
while (read(cin, student)) {
did.push_back(student);
}
// do the analyses
write_analysis(cout, "median", median_analysis, did, didnt);
return 0;
}
主文件
// grade.cpp
// compute a student's overall grade from midterm and final exam grades
// and homework grade
double grade(double midterm, double final, double homework)
{
return 0.2 * midterm + 0.4 * final + 0.4 * homework;
}
double grade_aux(const Student_info& s)
{
try {
return grade(s);
} catch (domain_error) {
return grade(s.midterm, s.final, 0);
}
}
等级.cpp
错误信息是:
g++ -Wall *.cpp -o main /tmp/ccbXbVcV.o: 在函数中
median_analysis(std::vector<Student_info, std::allocator<Student_info> > const&)': analysis.cpp:(.text+0x91): undefined reference to
double generic_analysis<&(grade_aux(Student_info const&))>(std::vector > const&)' collect2: ld 返回 1 退出状态
同时,当我将generic_analysis代码声明和定义放在头文件中时,它将起作用。请有人解释为什么?
编辑: 删除行时:
template<> double generic_analysis<average_grade>(const vector<Student_info>& students);
我有:
g++ -Wall *.cpp -o main /tmp/cc3VhXKU.o: 在函数中
median_analysis(std::vector<Student_info, std::allocator<Student_info> > const&)': analysis.cpp:(.text+0x91): undefined reference to
double generic_analysis<&(grade_aux(Student_info const&))>(std::vector > const&)' collect2: ld 返回 1 退出状态
所以好像我需要一个Explicit instantiation
. 然后我添加了以下行:
template double generic_analysis<grade_aux>(const vector<Student_info>&);
现在错误消息是:
g++ -Wall *.cpp -o 主
analysis.cpp: 在'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) =grade_aux]'的实例化中:
analysis.cpp:26:72: 从这里实例化 analysis.cpp:26:72: 错误: 'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) =grade_aux]' 的显式实例化但是没有可用的定义 [-fpermissive] analysis.cpp:在 'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) = grade_aux]' 的实例化中:
analysis.cpp:26:72: 从这里实例化 analysis.cpp:26:72: 错误: 'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) =grade_aux]' 的显式实例化但是没有可用的定义 [-fpermissive] analysis.cpp:在 'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) = grade_aux]' 的实例化中:
analysis.cpp:26:72: 从这里实例化 analysis.cpp:26:72: 错误: 'double generic_analysis(const std::vector&) [with double (* F)(const Student_info&) =grade_aux]' 的显式实例化但是没有可用的定义 [-fpermissive]