输出的差异取决于 for 循环中大括号的省略（c ++）[重复]

Question

按照本网站上的参考指南的建议，我一直在阅读 c++ 入门书，我注意到作者省略了 for 循环的花括号。我检查了其他网站，通常应该放入花括号。放大括号和省略时有不同的输出。代码如下

int sum = 0;
for (int val = 1; val <= 10; ++val)
    sum += val;  
    std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;

// This pair of code prints the std::cout once
for (int val = 50; val <=100;++val)
    sum += val;
    std::cout << "Sum of 50 to 100 inclusive is " << sum << std::endl;

// -------------------------------------------------------------------------
for (int val = 1; val <= 10; ++val) {
    sum += val;  
    std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

// This pair of code prints the std::cout multiple times
for (int val = 50; val <=100;++val) {
    sum += val;
    std::cout << "Sum of 50 to 100 inclusive is " << sum << std::endl;
}

如果有人能解释输出的差异，我将不胜感激。提前致谢！

Answer 1

for语句具体定义如下

for ( for-init-statement conditionopt; expressionopt) statement
                                                      ^^^^^^^^^

语句可以是任何语句，包括复合语句

compound-statement:
    { statement-seqopt}

因此在这个 for 语句的例子中

for (int val = 1; val <= 10; ++val)
sum += val; 

声明是

sum += val; 

在这个 for 语句的例子中

for (int val = 1; val <= 10; ++val) {
sum += val;  
std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

声明是

{
sum += val;  
std::cout << "Sum of 1 to 10 inclusive is " << sum << std::endl;
}

Answer 2

使用花括号，花括号中的所有内容都由 for 循环执行

for (...;...;...)
{
     // I get executed in the for loop!
     // I get executed in the for loop too!
}
// I don't get executed in the for loop!

但是，如果没有花括号，它只会直接执行它之后的语句：

for (...;...;...)
     // I get executed in the for loop!
// I don't get executed in the for loop!
// I don't get executed in the for loop either!