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I am a bit baffled that I managed to accidentally write code equivalent to this one
int a=a; // there is not a declared before this line
and the compiler happily compiled it - both gcc and clang, which are very standard-conforming and have good diagnostics. (With -Wall, gcc 4.8 warns about uninitialized variable; clang does not).
I thought the RHS of the assignment will be evaluated prior do LHS, hence causing a to be undefined on the RHS. Can I have some simple clarification about why is this syntactically legal?
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Initialization of static variables happens in two consecutive stages: static and dynamic initialization. Static initialization happens first and usually at compile time. If possible, initial values for static variables are evaluated during compilation and burned into the data section of the executable.
The reason is that your are defining your imageSegment outside of a function in your source code (static variable). In such cases, the initialization cannot include execution of code, like calling a function or allocation a class. Initializer must be a constant whose value is known at compile time.
An uninitialized variable is a variable that has not been given a value by the program (generally through initialization or assignment). Using the value stored in an uninitialized variable will result in undefined behavior.
Initializing a variable as Telastyn pointed out can prevent bugs. If the variable is a reference type, initializing it can prevent null reference errors down the line. A variable of any type that has a non null default will take up some memory to store the default value.
It will be a compile-time error if you tell GCC to make it so:
gcc -Winit-self -Werror
Note that sadly this diagnostic is not enabled by most of the usual suspects like -Wall.
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