构造函数创建const对象

为什么我得到错误

../implicit-explicit.cpp:44:10: error: invalid initialization of non-const reference of type ‘BWSize&’ from an rvalue of type ‘char’
../implicit-explicit.cpp:36:6: error: in passing argument 1 of ‘void func(BWSize&)’

当我从函数void func(const BWSize & s) 中删除const时

根据我的理解，构造函数返回的不是常量值。

代码

// implicit-explicit.cpp by Bill Weinman <http://bw.org/>
#include <iostream>
const std::size_t maxlen = 1024;    // maximum length of bwString
class BWSize {
    std::size_t _size;
public:
    BWSize(std::size_t);    // constructor: size from int
    BWSize(const char *);   // constructor: size from c-string
    std::size_t size() const;
};
BWSize::BWSize(const std::size_t n) {
    std::cout << "BWSize from int" << std::endl;
    _size = (n <= maxlen) ? n : 0;
}
BWSize::BWSize(const char * s) {
    std::cout << "constructor: BWSize from c-string" << std::endl;
    for(std::size_t i = 0; i < maxlen; i++) {
        if(s[i] == '') {
            _size = i;
            return;
        }
    }
    _size = 0;
}
std::size_t BWSize::size() const {
    return _size;
}
using namespace std;
void func(const BWSize & s) {
    cout << "s.size() is " << s.size() << endl;
}
int main( int argc, char ** argv ) {
    BWSize s = 'x';
    cout << "s.size() is " << s.size() << endl;
    func('x');
    return 0;
}