为什么a<<=b答案是4呢

int a = 3;
int b = 2;
cout << "a += b: " << (a += b) << endl;
cout << "a -= b: " << (a -= b) << endl;
cout << "a *= b:" << (a *= b) << endl;
cout << "a /= b: " << (a /= b) << endl;
cout << "a %= b: " << (a %= b) << endl;
cout << "a <<= b: " << (a <<= b) << endl;
cout << "a >>= b: " << (a >>= b) << endl;
cout << "a &= b: " << (a &= b) << endl;
cout << "a ^= b: " << (a ^= b) << endl;
cout << "a |= b: " << (a |= b) << endl;
return 0
为什么a<<=b答案是4呢

a+=b; 相当于 a= a+b;
a<<=b; 相当于 a = a<<b;
由于经过计算,a和b都已经改变了。

相当于 a = 1<<2;
1左移2 就是 4