Main.cc: In function ‘int main()’: Main.cc:7:13: error: no match for ‘operator==’ (operand types are ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ and ‘int’) if(k==1){ ^ In file included from /usr/include/c++/5/iosfwd:40:0, from /usr/include/c++/5/ios:38, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/postypes.h:216:5: note: candidate: template<class _StateT> bool std::operator==(const std::fpos<_StateT>&, const std::fpos<_StateT>&) operator==(const fpos<_StateT>& __lhs, const fpos<_StateT>& __rhs) ^ /usr/include/c++/5/bits/postypes.h:216:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::fpos<_StateT>’ if(k==1){ ^ In file included from /usr/include/c++/5/bits/stl_algobase.h:64:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/stl_pair.h:214:5: note: candidate: template<class _T1, class _T2> constexpr bool std::operator==(const std::pair<_T1, _T2>&, const std::pair<_T1, _T2>&) operator==(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y) ^ /usr/include/c++/5/bits/stl_pair.h:214:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::pair<_T1, _T2>’ if(k==1){ ^ In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/stl_iterator.h:292:5: note: candidate: template<class _Iterator> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&) operator==(const reverse_iterator<_Iterator>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:292:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::reverse_iterator<_Iterator>’ if(k==1){ ^ In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/stl_iterator.h:342:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&) operator==(const reverse_iterator<_IteratorL>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:342:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::reverse_iterator<_Iterator>’ if(k==1){ ^ In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/stl_iterator.h:1065:5: note: candidate: template<class _IteratorL, class _IteratorR> bool std::operator==(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&) operator==(const move_iterator<_IteratorL>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:1065:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::move_iterator<_Iterator>’ if(k==1){ ^ In file included from /usr/include/c++/5/bits/stl_algobase.h:67:0, from /usr/include/c++/5/bits/char_traits.h:39, from /usr/include/c++/5/ios:40, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/stl_iterator.h:1071:5: note: candidate: template<class _Iterator> bool std::operator==(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&) operator==(const move_iterator<_Iterator>& __x, ^ /usr/include/c++/5/bits/stl_iterator.h:1071:5: note: template argument deduction/substitution failed: Main.cc:7:15: note: ‘std::__cxx11::string {aka std::__cxx11::basic_string<char>}’ is not derived from ‘const std::move_iterator<_Iterator>’ if(k==1){ ^ In file included from /usr/include/c++/5/string:41:0, from /usr/include/c++/5/bits/locale_classes.h:40, from /usr/include/c++/5/bits/ios_base.h:41, from /usr/include/c++/5/ios:42, from /usr/include/c++/5/ostream:38, from /usr/include/c++/5/iostream:39, from Main.cc:1: /usr/include/c++/5/bits/allocator.h:128:5: note: candidat
错误信息 "no match for ‘operator==’ (operand types are ‘std::__cxx11::string {aka std::__cxx11::basic_string==
运算符来比较一个 std::string
类型的对象和一个 int
类型的值,但是编译器找不到合适的 operator==
来处理这两种不同的类型2。在 C++ 中,std::string
类型没有隐式地定义与 int
类型进行比较的操作符,因此不能直接进行比较。
要解决这个问题,你需要确保比较操作符的两边是相同或兼容的类型。如果你想要比较 std::string
对象和 int
,你可能需要将 std::string
对象转换为 int
,或者反过来,这取决于你的具体需求。例如,如果你想要检查一个字符串是否表示一个特定的数值,你可以使用 std::stoi
函数将字符串转换为 int
,然后进行比较:
std::string str = "123";
int num = 123;
if (std::stoi(str) == num) {
// 比较逻辑
}
另外,如果你想要比较字符串是否以特定的前缀或后缀开始或结束,你可以使用 std::string
的 substr
成员函数或 std::string::find
成员函数来进行比较。
请注意,C++20 引入了空间ship 操作符 <=>
,它提供了一种新的方式来定义比较操作符,这可能会影响到 operator==
的行为,但在这个具体的错误中并不适用4。此外,std::basic_string
的元素是连续存储的,这意味着你可以通过指针算术来访问它们56。然而,这与解决你当前的错误没有直接关系。
如何正确地使用C++中的类型转换运算符operator int()?
类型转换运算符operator int()
在C++中允许一个类的对象被转换成int
类型。这种转换是通过在类定义中添加一个特定的成员函数来实现的,该成员函数使用关键字operator
后跟int
来声明。根据1,这个函数的返回值类型就是函数名,因此不需要显式地表示出来。这意味着当你有一个类并希望对象能够被当作int
类型使用时,你只需要在类中定义一个operator int()
函数,编译器就会知道如何进行转换。例如,如果你有一个表示分数的类,并且你想能够通过调用operator int()
来获取分数的整数部分,你可以这样实现:
class Fraction {
int numerator, denominator;
public:
Fraction(int num, int denom) : numerator(num), denominator(denom) {}
int operator int() { return numerator / denominator; }
};
在C++中,如何避免出现'no match for 'operator==''的错误?
在C++中,出现"no match for 'operator=='"错误通常是因为尝试使用==
运算符比较两个不兼容的类型。为了避免这种错误,需要确保比较操作的两个操作数类型是兼容的。根据2,当使用std::string
类时,应该使用双引号""
来赋值,因为使用单引号''
或不加引号可能会导致类型不匹配的错误。例如,下面的代码是正确的:
std::string s;
s = "Hello World!";
如果使用单引号或不加引号,就可能触发"no match for 'operator=='"的错误。此外,如果需要比较自定义类型的对象,需要在类中重载operator==
以确保可以正确比较这些对象。15提到,如果使用了未定义的运算符,编译器会抛出这个错误,解决的办法是实现相应的运算符重载函数,并确保所定义的函数与使用的变量类型和数量相同。
为什么在C++中使用string类赋值时应该使用双引号而不是单引号?
在C++中,使用双引号来赋值给std::string
类是正确的做法,因为双引号表示字符串字面量,而单引号表示字符字面量。根据2,如果使用单引号或者不加引号给std::string
对象赋值,可能会出现类型不匹配的错误,如"no match for 'operator=='"。这是因为std::string
类型期望一个字符串类型的值,而不是单个字符。例如,正确的赋值方式是:
std::string s;
s = "Hello World!";
如果使用单引号:
s = 'H'; // 错误:尝试将字符赋值给字符串
这将导致编译错误,因为std::string
需要一个字符串,而不是一个字符。此外,21提到字符串字面值会隐式转换为std::string
对象,然后再赋给字符串变量,这进一步说明了使用双引号的重要性。
C++中operator==运算符重载为类成员函数时需要注意什么?
在C++中重载operator==
作为类成员函数时,需要注意几个关键点。首先,重载的操作符应该在类体内声明,并且其声明方式与普通成员函数类似,只是名称中包含关键字operator
和紧随其后的预定义操作符。例如,可以这样声明一个==
操作符:
class Person {
private:
int age;
public:
Person(int a) { this->age = a; }
inline bool operator==(const Person &ps) const;
};
然后,需要在类外实现这个操作符函数:
inline bool Person::operator==(const Person &ps) const {
return this->age == ps.age;
}
根据3,这样的实现允许使用==
运算符来比较两个Person
对象的年龄是否相等。此外,当重载operator==
时,还应该考虑操作符的对称性和重载其他相关的比较运算符,以确保比较操作的一致性和正确性。
C++20中比较运算符的新规则有哪些变化?
C++20引入了一些关于比较运算符的新规则,这些变化主要涉及重写和反向候选的概念。根据4,这意味着查找表达式如a == b
也将匹配对应的操作符,例如b == a
。这通常意味着开发者需要编写更少