增量声明的目的是什么？

它有时不仅仅是方便.

这些是等价的:

for (int a = 10; a < 20;  a = a + 1) {
    cout << a << endl;
}

for (int a = 10; a < 20;  ) {
    cout << a << endl;
    a = a + 1;
}

但是,这些不是:

// this works ...
for (int a = 10; a < 20;  a = a + 1) {
    if (blah ...)
        continue;
    cout << a << endl;
}

// this doesn't
for (int a = 10; a < 20;  ) {
    if (blah ...)
        continue;
    cout << a << endl;
    a = a + 1;
}

因为你是来自python,一个惯用的 for循环就像一个python range,但更强大.用Cthon表示的C for循环将是:

for a in range(10,20,1)

将此表达为以下内容更为惯用:

for (a = 10;  a < 20;  a += 1)

因为循环增量是1,所以这样做更加惯用:

for (a = 10;  a < 20;  ++a)

但是,for循环是:

for ([init_stmt];  [test_stmt];  [incr_stmt])

任何*_stmt都可以复合:

for (x = 0, y = 0;  x < 10;  ++x, y += 2)