Why doesn't c++ have &&= or ||= for booleans?

问题

Is there a "very bad thing" that can happen &&= and ||= were used as syntactic sugar for bool foo = foo && bar and bool foo = foo || bar?

回答1:

A bool may only be true or false in C++. As such, using &= and |= is relatively safe (even though I don’t particularly like the notation). True, they will perform bit operations rather than logical operations (and thus they won’t short-circuit) but these bit operations follow a well-defined mapping, which is effectively equivalent to the logical operations, as long as both operands are of type bool.¹

Contrary to what other people have said here, a bool in C++ must never have a different value such as 2. When assigning that value to a bool, it will be converted to true as per the standard.

The only way to get an invalid value into a bool is by using reinterpret_cast on pointers:

int i = 2;
bool b = *reinterpret_cast<bool*>(&i);
b |= true; // MAY yield 3 (but doesn’t on my PC!)

But since this code results in undefined behaviour anyway, we may safely ignore this potential problem in conforming C++ code.

---

¹ Admittedly this is a rather big caveat as Angew’s comment illustrates:

bool b = true;
b &= 2; // yields `false`.

The reason is that b & 2 performs integer promotion such that the expression is then equivalent to static_cast<int>(b) & 2, which results in 0, which is then converted back into a bool. So it’s true that the existence of an operator &&= would improve type safety.

回答2:

&& and & have different semantics: && will not evaluate the second operand if the first operand is false. i.e. something like

flag = (ptr != NULL) && (ptr->member > 3);

is safe, but

flag = (ptr != NULL) & (ptr->member > 3);

is not, although both operands are of type bool.

The same is true for &= and |=:

flag = CheckFileExists();
flag = flag && CheckFileReadable();
flag = flag && CheckFileContents();

will behave differently than:

flag = CheckFileExists();
flag &= CheckFileReadable();
flag &= CheckFileContents();

回答3:

Short answer

All the operators +=, -=, *=, /=, &=, |=... are arithmetic and provide same expectation:

x &= foo()  // We expect foo() be called whatever the value of x

However, operators &&= and ||= would be logical, and these operators might be error-prone because many developers would expect foo() be always called in x &&= foo().

bool x;
// ...
x &&= foo();           // Many developers might be confused
x = x && foo();        // Still confusing but correct
x = x ? foo() : x;     // Understandable
x = x ? foo() : false; // Understandable
if (x) x = foo();      // Obvious

• Do we really need to make C/C++ even more complex to get a shortcut for x = x && foo()?

• Do we really want to obfuscate more the cryptic statement x = x && foo()?
  Or do we want to write meaningful code like if (x) x = foo();?

---

Long answer

Example for &&=

If &&= operator was available, then this code:

bool ok = true; //becomes false when at least a function returns false
ok &&= f1();
ok &&= f2(); //we may expect f2() is called whatever the f1() returned value

is equivalent to:

bool ok = true;
if (ok) ok = f1();
if (ok) ok = f2(); //f2() is called only when f1() returns true

This first code is error-prone because many developers would think f2() is always called whatever the f1() returned value. It is like writing bool ok = f1() && f2(); where f2() is called only when f1() returns true.

• If the developer actually wants f2() to be called only when f1() returns true, therefore the second code above is less error-prone.
• Else (the developer wants f2() to be always called), &= is sufficient:

Example for &=

bool ok = true;
ok &= f1();
ok &= f2(); //f2() always called whatever the f1() returned value

Moreover, it is easier for compiler to optimize this above code than that below one:

bool ok = true;
if (!f1())  ok = false;
if (!f2())  ok = false;  //f2() always called

Compare && and &

We may wonder whether the operators && and & give the same result when applied on bool values?

Let's check using the following C++ code:

#include <iostream>

void test (int testnumber, bool a, bool b)
{
   std::cout << testnumber <<") a="<< a <<" and b="<< b <<"\n"
                "a && b = "<< (a && b)  <<"\n"
                "a &  b = "<< (a &  b)  <<"\n"
                "======================"  "\n";
}

int main ()
{
    test (1, true,  true);
    test (2, true,  false);
    test (3, false, false);
    test (4, false, true);
}

Output:

1) a=1 and b=1
a && b = 1
a &  b = 1
======================
2) a=1 and b=0
a && b = 0
a &  b = 0
======================
3) a=0 and b=0
a && b = 0
a &  b = 0
======================
4) a=0 and b=1
a && b = 0
a &  b = 0
======================

Conclusion

Therefore YES we can replace && by & for bool values ;-)
So better use &= instead of &&=.
We can consider &&= as useless for booleans.

Same for ||=

operator |= is also less error-prone than ||=

If a developer wants f2() be called only when f1() returns false, instead of:

bool ok = false;
ok ||= f1();
ok ||= f2(); //f2() is called only when f1() returns false
ok ||= f3(); //f3() is called only when f1() or f2() return false
ok ||= f4(); //f4() is called only when ...

I advice the following more understandable alternative:

bool ok = false;
if (!ok) ok = f1();
if (!ok) ok = f2();
if (!ok) ok = f3();
if (!ok) ok = f4();
// no comment required here (code is enough understandable)

or if you prefer all in one line style:

// this comment is required to explain to developers that 
// f2() is called only when f1() returns false, and so on...
bool ok = f1() || f2() || f3() || f4();

来源：https://stackoverflow.com/questions/2488406/why-doesnt-c-have-or-for-booleans