如何将一个数字转换为在sql server中使用0前缀的字符串(How to cast a number to string with 0 prefix in sql server)

 此问题是由构造函数中的隐式转换引起的。 字符串文字 （例如代码中的字符串文字）存储为const char类型。 因为您没有采用此类型的构造函数，所以编译器会尝试查找转换为可在其中一个构造函数中找到的类型。 
 
 在这种情况下， const char*转换为bool QString更容易，所以当你这样做时： 
 
DB_Variable foo("some_name"," ");
 
 构造函数 
 
DB_Variable(QString name, bool newValue):
 
 叫做。 
 
 请注意，您看到的行为不是因为" "得到的处理方式与任何其他字符串文字不同，只是因为您很可能没有类型为bool, bool的构造函数（所有构造函数都将QString作为第一个论点？）。 如果你有一个如下构造函数，你可能会： 
 
DB_Variable(bool test1, bool newValue):
 
 当你做了一些像DB_Variable foo("some_name"," ");这样的事情时，就会调用它DB_Variable foo("some_name"," "); 
 
 要获得您想要的结果，您可以传递QStrings如下所示： 
 
DB_Variable foo(QString("some_name"), QString());
 
 或者也许定义一个构造函数，它将const char*作为第二个参数。 

This problem results from implicit conversions going on in the constructor. String literals, such as the one in your code, are stored as const char types. Because you didn't have a constructor taking this type the compiler tries to find the conversion to a type that it can find in one of your constructors.
 
In this case const char* converts to bool easier that QString so when you do:
 
DB_Variable foo("some_name"," ");
 
The constructor 
 
DB_Variable(QString name, bool newValue):
 
Is called.
 
Note that the behavior you are seeing isn't due to " " getting treated differently than any other string literal, it's just that you most likely didn't have a constructor with the types bool, bool (did all your constructors take a QString as the first argument?). Chances are if you had a constructor such as the following:
 
DB_Variable(bool test1, bool newValue):
 
Then this would have been called instead when you did something such as DB_Variable foo("some_name"," ");
 
To get the results you wanted you could pass in QStrings like so:
 
DB_Variable foo(QString("some_name"), QString());
 
Or perhaps define a constructor that takes const char* for the second parameter.