C++ 从“const char*”到“char*”的无效转换
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invalid conversion from 'const char*' to 'char*'
提问by user2333234
Have a code as shown below. I have problem passing the arguments.
有一个代码，如下所示。我在传递参数时遇到问题。
stringstream data;
char *addr=NULL;
strcpy(addr,retstring().c_str());
retstring()is a function that returns a string.
retstring()是一个返回字符串的函数。
//more code
printfunc(num,addr,data.str().c_str());
I get the error
我收到错误
invalid conversion from 'const char*' to 'char*'.
initializing argument 3 of 'void Printfunc(int, char*, char*)'on argument 3 of the function
从“const char*”到“char*”的无效转换。
在函数的参数 3 上初始化“void Printfunc(int, char*, char*)”的参数 3
on the above line. The function is called as shown below
在上面的线上。函数调用如下图
void Printfunc(int a, char *loc, char *stream)
please let me know if I need to change any initialization.
如果我需要更改任何初始化，请告诉我。
回答by Dietmar Kühl
Well, data.str().c_str()yields a char const*but your function Printfunc()wants to have char*s. Based on the name, it doesn't change the arguments but merely prints them and/or uses them to name a file, in which case you should probably fix your declaration to be
好吧，data.str().c_str()产生 achar const*但你的函数Printfunc()想要有char*s。根据名称，它不会更改参数，而只是打印它们和/或使用它们来命名文件，在这种情况下，您可能应该将声明修改为
void Printfunc(int a, char const* loc, char const* stream)
The alternative might be to turn the char const*into a char*but fixing the declaration is preferable:
另一种方法可能是将 thechar const*变成 achar*但更可取的是修复声明：
Printfunc(num, addr, const_cast<char*>(data.str().c_str()));
回答by Louis93
string::c.str()returns a string of type const char *as seen here
string::c.str()返回一个类型的字符串，const char *如这里所示
A quick fix: try casting printfunc(num,addr,(char *)data.str().c_str());
快速修复：尝试铸造printfunc(num,addr,(char *)data.str().c_str())；
While the above may work, it is undefined behaviour, and unsafe.
虽然上述可能有效，但它是未定义的行为，并且不安全。
Here's a nicer solution using templates:
这是使用模板的更好的解决方案：
char * my_argument = const_cast<char*> ( ...c_str() );
回答by Vlad from Moscow
First of all this code snippet
首先这个代码片段
char *addr=NULL;
strcpy(addr,retstring().c_str());
is invalid because you did not allocate memory where you are going to copy retstring().c_str().
无效，因为您没有在要复制 retstring().c_str() 的地方分配内存。
As for the error message then it is clear enough. The type of expression data.str().c_str() is const char * but the third parameter of the function is declared as char *. You may not assign an object of type const char * to an object of type char *. Either the function should define the third parameter as const char * if it does not change the object pointed by the third parameter or you may not pass argument of type const char *.
至于错误信息，那就很清楚了。表达式 data.str().c_str() 的类型是 const char * 但函数的第三个参数被声明为 char *。您不能将 const char * 类型的对象分配给 char * 类型的对象。如果函数不更改第三个参数指向的对象，则该函数应将第三个参数定义为 const char *，或者您可能未传递类型为 const char * 的参数。
