std :: string構造在參數

Question

我遇到了一個非常棘手的C++編譯器錯誤。

當我構造一個字符串作爲參數使用時，它在調用常規方法時起作用。例如。 printThisString(string(charPtr));

構建對象時，如果構造函數的參數是char*，它不起作用。例如，MyObject a(string(argv[0]));如果參數是文字，它仍然有效。例如，MyObject b(string("hi"));

#include <iostream> 
#include <string> 

using namespace std; 

void printString(string toPrint) { 
    cout << toPrint << endl; 
} 

class MyObject { 
    int blah; 
public: 
    void aMethod() {} 
    MyObject (string myStr) { 
     cout << myStr << endl; 
    } 
}; 

int main(int argc, char ** argv) { 

string s1(argv[0]); 
char * s2 = "C-style string"; // I realize this is bad style 

printString(string("Hello world!")); // All of these work 
printString(s1); 
printString(string(s2)); 
printString(string(argv[0])); 

MyObject mo1 (string("Hello world!")); // Valid 
MyObject mo2 (s1); // Valid 
MyObject mo3 (string(s2)); // Does not print 
MyObject mo4 (string(argv[0])); // Does not print 

mo1.aMethod(); 
mo2.aMethod(); 
mo3.aMethod(); // Error 
mo4.aMethod(); // Error 


return 0; 
} 

對於M03和MO4，對象可以被創建，但也可以使用任何方法。他們是錯誤的典型。看來，編譯器認爲他們是功能...

test.cpp: In function 'int main(int, char**)': 
test.cpp:22:13: warning: deprecated conversion from string constant to 'char*' [-Wwrite-strings] 
test.cpp:36:5: error: request for member 'aMethod' in 'mo3', which is of non-class type 'MyObject(std::string) {aka MyObject(std::basic_string<char>)}' 
test.cpp:37:5: error: request for member 'aMethod' in 'mo4', which is of non-class type 'MyObject(std::string*) {aka MyObject(std::basic_string<char>*)}' 

Answer 1

這只是一個most vexing parse的變化：mo3和mo4是函數聲明，而不是對象定義。您可以修復使用

MyObject mo3 {string(s2)}; 
MyObject mo4 {string(argv[0])}; 

或

MyObject mo3 ((string(s2))); 
MyObject mo4 ((string(argv[0]))); 

或

MyObject mo3 = MyObject(string(s2)); 
MyObject mo4 = MyObject(string(argv[0])); 

或...