今天在工作的时候,我在C++中遇到了一个我不理解的行为。我已经生成了以下示例代码来说明我的问题:
#include <string>
#include <iostream>
class MyException
{
public:
MyException(std::string s1) {std::cout << "MyException constructor, s1: " << s1 << std::endl;}
};
int main(){
const char * text = "exception text";
std::cout << "Creating MyException object using std::string(const char *)." << std::endl;
MyException my_ex(std::string(text));
std::cout << "MyException object created." << std::endl;
//throw my_ex;
std::string string_text("exception text");
std::cout << "Creating MyException object using std::string." << std::endl;
MyException my_ex2(string_text);
std::cout << "MyException object created." << std::endl;
// throw my_ex2;
return 0;
}
此代码段编译时没有任何错误,并生成以下输出:
$ g++ main.cpp
$ ./a.out
Creating MyException object using std::string(const char *).
MyException object created.
Creating MyException object using std::string.
MyException constructor, s1: exception text
MyException object created.
注意,对于my_ex
,我定义的构造函数没有被调用。接下来,如果我真的想抛出这个变量:
throw my_ex;
我得到一个编译错误:
$ g++ main.cpp
/tmp/ccpWitl8.o: In function `main':
main.cpp:(.text+0x55): undefined reference to `my_ex(std::string)'
collect2: error: ld returned 1 exit status
如果我在转换前后添加大括号,如下所示:
const char * text = "exception text";
std::cout << "Creating MyException object using std::string(const char *)." << std::endl;
MyException my_ex((std::string(text)));
std::cout << "MyException object created." << std::endl;
throw my_ex;
然后它就会像我预期的那样工作:
$ g++ main.cpp
$ ./a.out
Creating MyException object using std::string(const char *).
MyException constructor, s1: exception text
MyException object created.
terminate called after throwing an instance of 'MyException'
Aborted (core dumped)
我有以下问题:
throw my_ex;
发布于 2016-11-23 23:01:57
根据most vexing parse的说法,MyException my_ex(std::string(text));
是一个函数声明;该函数名为my_ex
,接受类型为std::string
的名为text
的参数,返回MyException
。它根本不是一个对象定义,因此不会调用任何构造函数。
注意throw my_ex;
的错误消息undefined reference to 'my_ex(std::string)'
(实际上您试图抛出一个函数指针),这意味着找不到函数my_ex
的定义。
要修复它,您可以添加其他括号(如您所示)或使用C++11支持的braces:
MyException my_ex1((std::string(text)));
MyException my_ex2{std::string(text)};
MyException my_ex3{std::string{text}};
发布于 2016-11-24 03:31:54
答案是尽可能多地使用{}
(带括号的-init)。然而,有时可能会无意中错过它。幸运的是,编译器(如clang,没有额外的警告标志)可以提示:
warning: parentheses were disambiguated as a function declaration [-Wvexing-parse]
MyException my_ex(std::string(text));
^~~~~~~~~~~~~~~~~~~
test.cpp:13:23: note: add a pair of parentheses to declare a variable
MyException my_ex(std::string(text));
^
( )
1 warning generated.
这将立即为您指出问题所在。
https://stackoverflow.com/questions/40767681
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