temporary

Are temporary objects in C++ const indeed?

由 淺唱寂寞╮ 提交于 2019-12-09 00:54:03
问题 I always believed that temporary objects in C++ are automatically considered as const by the compiler. But recently I experienced that the following example of code: function_returning_object().some_non_const_method(); is valid for C++ compiler. And it makes me wonder - are temporary objects in C++ const indeed? If yes, then why the code above is considered correct by the compiler? 回答1: No, they're not. Not unless you declare the return type as const. 回答2: It depends. int f(); const int g();

invalid initialization of non-const reference of type 'int&' from a temporary of type 'int'

由 China☆狼群 提交于 2019-12-08 17:24:37
问题 #include<iostream> using namespace std; int fun(int &x) { return x; } int main() { cout << fun(10); return 0; } Can anyone explain the reason of the error ? Thanks 回答1: 10 is a constant, so you can't pass a reference to it, simply because the whole concept of changing a constant is bizarre. References were introduced to solve one of the thorny problems in C (and earlier C++), the fact that everything is passed by value and, if you want to have a change reflected back to the caller, you have

MySQL - INSERT INTO from a Temporary Table

由 本秂侑毒 提交于 2019-12-07 22:43:03
问题 This seems stupidly easy but I'm stuck with a simple insert statement.See below: begin work; CREATE TEMPORARY TABLE IF NOT EXISTS insert_table AS ( select r.resource_id ,fr.file_repos_id ,mv.VALUE from resources r join versions v on v.RESOURCE_ID = r.resource_id join metadata_values mv on mv.resource_id = r.resource_id join file_repository fr on fr.file_repos_id = v.foreign_id where v.version_status = 'C' and r.RESOURCE_TYPE = 4 and fr.file_title in ('suburbs') ); insert into metadata_values

Temporary Table - Maximum allowed number of 1000 row values

由 ∥☆過路亽．° 提交于 2019-12-07 06:13:35
问题 When trying to insert 6000 rows into a temp table I get the following message The number of row value expressions in the INSERT statement exceeds the maximum allowed number of 1000 row values. Source is not located in SQL Server . CREATE TABLE #TMP_ISIN ( [Isin] nVARCHAR(250)) INSERT INTO #TMP_ISIN ([Isin]) VALUES ABOUT 6000 ROWS How shall I do to avoid this limit? 回答1: The limit of 1000 is on the number of rows in the values clause of the insert rather than a limitation of the temporary

Fix Using where; Using temporary; Using filesort

由 天大地大妈咪最大 提交于 2019-12-07 03:46:32
问题 I have two simple tables: CREATE TABLE cat_urls ( Id int(11) NOT NULL AUTO_INCREMENT, SIL_Id int(11) NOT NULL, SiteId int(11) NOT NULL, AsCatId int(11) DEFAULT NULL, Href varchar(2048) NOT NULL, ReferrerHref varchar(2048) NOT NULL DEFAULT '', AddedOn datetime NOT NULL DEFAULT CURRENT_TIMESTAMP, GroupId int(11) DEFAULT NULL, PRIMARY KEY (Id), INDEX SIL (SIL_Id, AsCatId) ) CREATE TABLE products ( Id int(11) NOT NULL AUTO_INCREMENT, CatUrlId int(11) NOT NULL, Href varchar(2048) NOT NULL,

prohibiting instantiation as a temporary object (C++)

由 六月ゝ 毕业季﹏ 提交于 2019-12-06 20:17:06
问题 I like using sentry classes in c++, but I seem to have a mental affliction that results in repeatedly writing bugs like the following: { MySentryClass(arg); // ... other code } Needless to say, this fails because the sentry dies immediately after creation, rather than at the end of the scope, as intended. Is there some way to prevent MySentryClass from being instantiated as a temporary, so that the above code either fails to compile, or at least aborts with an error message at runtime? 回答1: I

rules with temporary objects and args by reference

由 老子叫甜甜 提交于 2019-12-06 11:24:54
say I have a class: class A { public: A() {} }; and a function: void x(const A & s) {} and I do: x(A()); could someone please explain to me the rules regarding passing temporary objects by reference? In terms of what the compiler allows, where you need const, if an implicit copy happens, etc. From playing around, it seems like you need the const which makes sense, but is there a formal rule regarding all this? Thanks! There is a formal rule - the C++ Standard (section 13.3.3.1.4 if you are interested) states that a temporary can only be bound to a const reference - if you try to use a non

MySQL - INSERT INTO from a Temporary Table

由 扶醉桌前 提交于 2019-12-06 11:03:27
This seems stupidly easy but I'm stuck with a simple insert statement.See below: begin work; CREATE TEMPORARY TABLE IF NOT EXISTS insert_table AS ( select r.resource_id ,fr.file_repos_id ,mv.VALUE from resources r join versions v on v.RESOURCE_ID = r.resource_id join metadata_values mv on mv.resource_id = r.resource_id join file_repository fr on fr.file_repos_id = v.foreign_id where v.version_status = 'C' and r.RESOURCE_TYPE = 4 and fr.file_title in ('suburbs') ); insert into metadata_values (elem_id,value,resource_type,resource_id,foreign_id,mtvr_id,mett_id) values (62,'test',4,insert_table

Simple way to pass temporary struct by value in C++?

由 梦想的初衷 提交于 2019-12-05 20:20:13
问题 Suppose I want to pass a temporary object into a function. Is there a way to do that in 1 line of code vs. 2, with a struct? With a class, I can do: class_func(TestClass(5, 7)); given: class TestClass { private: int a; short b; public: TestClass(int a_a, short a_b) : a(a_a), b(a_b) { } int A() const { return a; } short B() const { return b; } }; void class_func(const TestClass & a_class) { printf("%d %d\n", a_class.A(), a_class.B()); } Now, how do I do that with a struct? The closest I've got

Fix Using where; Using temporary; Using filesort

由 落爺英雄遲暮 提交于 2019-12-05 07:24:58
I have two simple tables: CREATE TABLE cat_urls ( Id int(11) NOT NULL AUTO_INCREMENT, SIL_Id int(11) NOT NULL, SiteId int(11) NOT NULL, AsCatId int(11) DEFAULT NULL, Href varchar(2048) NOT NULL, ReferrerHref varchar(2048) NOT NULL DEFAULT '', AddedOn datetime NOT NULL DEFAULT CURRENT_TIMESTAMP, GroupId int(11) DEFAULT NULL, PRIMARY KEY (Id), INDEX SIL (SIL_Id, AsCatId) ) CREATE TABLE products ( Id int(11) NOT NULL AUTO_INCREMENT, CatUrlId int(11) NOT NULL, Href varchar(2048) NOT NULL, SiteIdentity varchar(2048) NOT NULL, Price decimal(12, 2) NOT NULL, IsAvailable bit(1) NOT NULL, ClientCode
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