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javac++bytecode

What is the diffrrent between Bytecode Instruction that makes one language support premitive null check inside if flow control and the other not?

why c++ support this 

int main()
{
  if(NULL)
    std::cout << "compile";  
  if(5)       
    std::cout << "compile";  
  if(4.78)
    std::cout << "compile";
  if("lala")
    std::cout << "compile";
}

and in java 

public class Test {
    public static void main(String[] args) {
        if(null) System.out.println("true");
    }
}

Error:(5, 12) java: incompatible types: cannot be converted to boolean

from https://en.wikipedia.org/wiki/Java_bytecode_instruction_listings

enter image description here

java and c++ it's just an example Lua and js also support this and kotlin not
I am trying to understand why one language support null check by default and the other doesn't?

Solution

  • They are different languages, and different languages can have different semantics! In C or C++, NULL actually just is a pre-processor macro, either defined to 0 or to an expression that produces a null pointer (((void*)0)). Additionally, C++ (but not C!) comes with a null pointer keyword (nullptr, since C++11) which nowadays should be preferred over the macro or the integer literal 0.

    Then in C and in C++, you can test numbers (integral or floating point) and pointers directly. Anything that compares equal to 0 is accepted as false, anything else is accepted as true. For instance:

    int n = 0;
int* p = nullptr; // C++ only, 0 or NULL in C
double d = 0.0;

if(n) { }
if(p) { }
if(d) { }
// all of these are equivalent to if(x != 0), x being n, p or d

    Alike, you can test literals as well (all below actually never enter the if branch):

    if(false) { }
if(0) { }
if(nullptr) { }

    Java, in contrast, only accepts true boolean values. So what is implicit in C or C++ must be written explicitly in Java (and possibly Lua, but I'm not familiar with):

    if(someBoolean) { }        // fine, as boolean already
if(someInt != 0) { }
if(someReference != null) { }

    Side note: There are yet some other languages that use C/C++ semantics as well, for instance Python or JavaScript.