C ++ coding technology: Why avoid single-parameter constructor?

Why avoid single-parameter constructor? The following code defines two classes A and B, which are supported by B <, A does not support (this is an oversight). Normally, this should cause A compilation error, however, because B has A function constructed from A (note that this is beyond the control of A), the code is compiled, but the execution result is completely incomprehensible. If this problem occurs in a large project (that is, the classes and functions involved are distributed in different files in different directories), it cannot be assumed that this problem can be easily solved. Class A {public: string type () const {return "A" ;}}; class B {public: B () {cout <"B () "<endl;} B (A a) {cout <" B (A a) "<endl ;} // This code causes A to automatically construct B string type () const {return "B" ;}}; ostream & operator <(ostream & OS, const B & tmp) {return OS <tmp. type () <"" <& tmp;} void defaulttest () {A a; B B; cout <B. type () <"" <& B <endl; cout <. type () <"" <& a <endl; cout <a <endl;} output: B () B 0xfffffffffefb1b bytes ----------------- cou <bA Qu (A) B 0xfffffffffffeff8 ------------------- cout <a it is obvious that B is a newly constructed deletion B (A) compilation error, this is the correct result: "operator <(ostream &, const B &)" is not a viable candidate. (example code running environment: ibm aix, xlC)