检查字符串是否为""

1） check non-empty string, true, 42, Infinity, [], ...

if (strValue) {

}

2) check empty string, false, 0, null, undefined, ...

if (!strValue) {

}

3) Check empty string only

if (strValue === "") {

}

4) Check non-empty string

if (strValue !== "") {

}

Comments

Templates

Template - Polymorphism is the ability of the same code to operate on different types. This ability to operate on multiple types reduces code duplication by allowing the same piece of code to be reused across the different types it can operate on. - Polymorphism comes in a variety of forms. What we are interested in at the moment is parametric polymorphism, meaning that we can write our code so that it is parameterized over what type it operates on. -That is, we want to declare a type parameter T and replace int with T in the above code. -Then, when we want to call the function, we can specify the type for T and get the function we desire. C++ provides parametric polymorphism through templates. Templated Functions - We can write a templated function by using the keyword template followed by the template parameters in angle brackets (<>). - Unlike function parameters, template parameters may be types, which are specified with typename where the type of the parameter wo...

useMemo的使用场景

useMemo是用来缓存计算属性的。计算属性是函数的返回值，或者说那些以返回一个值为目标的函数。有些函数会需要我们手动去点击，有些函数是直接在渲染的时候就执行，在DOM区域被当作属性值一样去使用。后者被称为计算属性。 e.g. const Component = () => { const [ params1 , setParams1 ] = useState ( 0 ); const [ params2 , setParams2 ] = useState ( 0 ); //这种是需要我们手动去调用的函数 const handleFun1 = () => { console . log ( "我需要手动调用，你不点击我不执行" ); setParams1 (( val ) => val + 1 ); }; //这种被称为计算属性，不需要手动调用，在渲染阶段就会执行的。 const computedFun2 = () => { console . log ( "我又执行计算了" ); return params2 ; }; return ( < div onClick = { handleFun1 } > //每次重新渲染的时候我就会执行 computed: { computedFun2 () } </ div > ); }; 上面的代码中，在每次点击div的时候，因为setParams1的缘故，导致params1改变，整个组件都需要重新渲染，也导致comptedFunc2()也需要重新计算。如果computedFunc2的计算量很大，这时候重新计算会比较浪费。可以使用useMemo: const Com = () => { const [ params1 , setParams1 ] = useState ( 0 ); const [ params2 , setParams2 ] = useState ( 0 ); //这种是需要我们手动去调用的函数 const handleFun1 ...

Valgrind

Using Valgrind to check memory How to use Valgrind -To valgrind the program, run the valgrind command and give it our program name as an argument. -For example, if we want to run ./myProgram hello 42, we can simply run Valgrind ./myProgram hello 42. Uninitialized Values -When we run the program, we may use uninitialized values. It needs to be fixed. Valgrind can tell us about the use of uninitialized values. But it only tell when the control flow of the program depends on the unitialized value. For example, uninitialized value appears in the conditional expression of an if, or a loop, or in the switch statement. -If we want to know where the uninitialized value is from, we can use Valgrind --track-origins=yes ./myProgram -Using -fsanitize=address can find a lot of problems that Memcheck cannot. -We can use Valgrind with GDB to debug. We can run: --vgdb=full --vgdb-error=0., then Valgrind will stop on the first error that it encounters and give control to ...

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