为什么我的const char * cast不能正常工作？ [英] Why isn't my const char* cast working correctly?

问题描述

我正在尝试通过C ++库将查询传递给MySQL。但是，查询需要const char *数据类型，所以我尝试了...

I''m trying to pass a query to MySQL via a C++ library. However, the query requires a const char* data type, so I tried...

//Initialization code ommitted.. includes declaration of conn, among others...

	const char* updateStr = ("update users set " + dbProperty + "='" + newValue + "' where id=" + ID + ";").c_str();
	mysql_query(conn, updateStr);

	mysql_close(conn);

dbProperty，newValue和ID是函数的所有参数。但是，我的代码仍然无法正常工作。我知道API设置正确，因为我可以使用mysql_query从数据库中提取数据而不会出现问题。它写的是'问题所在。上面的查询语法是正确的，因为我在服务器控制台窗口中尝试了它，并且它工作正常。我想确保char *正确添加，所以我尝试了..

dbProperty, newValue, and ID are all parameters of the function. However, my code still not working properly. I know that the API is set up correctly because I can use mysql_query to pull data from the database without an issue. It''s writing that''s the problem. The above query syntax is correct though because I tried it in the server console window, and it worked fine. I wanted to make sure that the char* was appending correctly so I tried..

std::cout << updateStr << std::endl;

所有我打印的都是破折号。这是怎么回事？为什么我不能构建一个字符串，将其转换为const char *并使用它？任何想法？

And all I got printed was a dash. What''s going on? Why can''t I build a string, convert it to const char*, and use it? Any ideas?

推荐答案

您正在构建一个临时的std :: string对象并询问它的起始地址（这就是c_str（）的作用）。由于字符串是临时的，一旦控件移动到下一个语句，它就会超出范围。因此，在执行mysql_query时，字符串内容已经是未定义的。



相反，将连接分配给std :: string对象：

You are building a temporary std::string object and ask it for its starting address (that''s what c_str() does). As the string is temporary it will go out of scope as soon as control moves to the next statement. So, by the time mysql_query is executed, the string contents is already undefined.

Instead, assign the concattenation to a std::string object:

std::string updateCommand = ....
mysql_query (conn, updateCommand.c_str());

这篇关于为什么我的const char * cast不能正常工作？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！