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How should one use std::optional?
...
@Rapptz Line 256: union storage_t { unsigned char dummy_; T value_; ... } Line 289: struct optional_base { bool init_; storage_t<T> storage_; ... } How is that not "a T and a bool"? I completely agree the implementation is very tricky an...
What's the right way to pass form element state to sibling/parent elements?
...ContextContainer = React.createContext(null);
const initialAppState = {
selected: "Nothing"
};
function App() {
// The app has a state variable and update handler
const [appState, updateAppState] = useState(initialAppState);
return (
<div>
<h1>Passing state between c...
Why does Java's hashCode() in String use 31 as a multiplier?
... for 31 since 33 is not a prime:
Of the remaining
four, I'd probably select P(31), as it's the cheapest to calculate on a RISC
machine (because 31 is the difference of two powers of two). P(33) is
similarly cheap to calculate, but it's performance is marginally worse, and
33 is composit...
Is short-circuiting logical operators mandated? And evaluation order?
... function, at least one of the arguments needs to be a class (or struct or union) or an enum, or a reference to one of those. Being POD means that you can use memcpy on it.
– Derek Ledbetter
Mar 12 '09 at 21:45
...
Bogus foreign key constraint fail
...Two possibilities:
There is a table within another schema ("database" in mysql terminology) which has a FK reference
The innodb internal data dictionary is out of sync with the mysql one.
You can see which table it was (one of them, anyway) by doing a "SHOW ENGINE INNODB STATUS" after the drop f...
ATL COM开发入门（一）（JS调用ActiveX/COM组件） - C/C++ - 清泛网 - 专注C/C++及内核技术
...它进行COM开发。本文通过一个基础入门的例子，简要介绍如何定义、实现C...ATL（Active Template Library）动态模板库，是一个类库，可以使用它进行COM开发。本文通过一个基础入门的例子，简要介绍如何定义、实现COM接口，前台如何...
MySQL: multiple tables or one table with many columns?
...
Recently I was faced with this same problem, because MySQL InnoDB tables have a relatively small length limit (~8000 bytes). In my problem table (data from very lengthy insurance forms, more than 100 columns) we have multiple varchar columns, all UTF8. So we easily filled the ~...
App Inventor 2 中文网原创内容 · App Inventor 2 中文网
...科普】App Inventor 2 添加超过10个屏幕会怎样？
【算法】如何计算1加到100的总和？（经典循环的写法）
【算法】App Inventor 2 算法之二分算法（Binary Search）实现，快速查找定位
组件用法
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An explicit value for the identity column in table can only be specified when a column list is used
...tead of
SET IDENTITY_INSERT archive_table ON;
INSERT INTO archive_table
SELECT *
FROM source_table;
SET IDENTITY_INSERT archive_table OFF;
you need to write
SET IDENTITY_INSERT archive_table ON;
INSERT INTO archive_table (field1, field2, ...)
SELECT field1, field2, ...
FROM source_table;...
Convert from MySQL datetime to another format with PHP
I have a datetime column in MySQL.
18 Answers
18
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