Handle declarations

Can anyone tell me what the difference is between these 2 lines 开发者_运维百科of code, which one is better to use?

    System::String ^MyStr = gcnew System::String(MyStr); 

    System::String ^MyStr; 

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Those lines are not equivalent. In the first one, you will get an exception beacuse you're trying to create a String from an uninitialized tracking handle (MyStr). In the second one, MyStr is declared, not defined, it points to garbage and will throw an exception if you attempt to use it. Which one you should use depends on the rest of the code

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The second one creates a new handle variable. If it's a local variable, then as @dario_ramos says, it's uninitialized, and your program will likely crash if you try to use the handle before assigning it. If it's a member variable or global, then it will be nullptr.

The first one is similar, although it can only be used for locals or globals (member variables use the ctor-initializer syntax in C++/CLI just like plain C++), and does exactly what you're not permitted to do. It reads the brand new uninitialized handle and passes it to the System::String constructor. If by chance the constructor finishes, a handle to the newly constructed String will be placed into the variable as part of initialization. But because the constructor is trying to make a copy of random garbage (if it's a local) or nullptr (if a global), most likely it will simply crash.

It's a bad idea to use the value of any variable in its own initializer (sometimes you need to use the address, never the value).