C ++：如何确保所有变量都已初始化？

Question

Recently I had lots of trouble with a non initialized variable.

In Java, the default value of variable is null, therefore an exception is likely to be thrown when if the non-initialized variable is used. If I understood, in C++, the variable is initialized with whatever data turns out to be in the memory. Which means that the program is likely to run, and it might be hard to even know there is something wrong with it.

What would be the clean way to deal with this ? Is there some good programming habit that would reduce the risk ? In my case, the variable was declared in the header file and should have been initialized in the cpp file, which is an example of things that makes error more likely.

thx

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Edition after receiving few answers:

My apologies, my question was not specific enough.

The answer I get to use flag for the compilers to get informed of non-initialized variables will be useful.

But there are rare cased variables can not be initialized at the beginning, because depending on the behavior of your system.

in header file

double learnedValue;

in cpp file

/* code that has nothing to do with learnedValue
...
*/

learnedValue = a*b*c; // values of a, b and c computed in the code above

/*code making use of learned value
...
*/

Now what happened is that forgot the line "learnedValue=a*b*c".

But the program was working good, just with value of learnedValue initialized with whatever what was in the memory when it was declared.

In Java, such error is not an issue, because the code making use of learned value is likely to crash or throw an exception (at least you get to know what was wrong).

In C++, you can apparently be happy and never get to know there is a problem at all. Or ?

Answer 1

Pls make sure you have appropriate warning levels set while compiling your program. Compilers issue appropriate warning whenever un-initialized variables are used.

On g++, -Wall compiler option would show all warnings.

On Visual studio, you might have to use warning level 4.

Also, there are some static code analysis tool available in the market. cppCheck is one such tool available for free.

Answer 2

You should not define a variable in a header (only declare it). Otherwise you will get other errors when you include the header in several .cpp files.

When actually defining a variable, you can also give it an initial value (like 0). In C++ it is also common to defer the definition of (local) variables until you have a value to assign to them.

In the header file

extern double learnedValue;
^^^^^^

In the cpp file

double learnedValue = 0;

/* code that has nothing to do with learnedValue
...
*/

learnedValue = a*b*c; // values of a, b and c computed in the code above

/*code making use of learned value
...
*/

Answer 3

您可以在声明变量的地方定义变量

Answer 4

c++11 allows you to initialize variables inside class. If that is not implemented by the compiler yet then the constructor initialization list is the area to check.

Answer 5

The C# can initialize the variable. But C++ not, so when use a pointer without initialized, it always throw exception. You should make a good habit to initialize all the variables in the class constructor.