No, this is not safe. More precisely this is UB, means anything is possible. When you pass 123.0 + 2.0 to the constructor of Foo, a temporary double will be constructed and bound to the parameter fx. The temporary will be destroyed after the full expression (i.e. Foo p(123.0 + 2.0);), then the reference member … Read more
I’ll answer your question… the other way around. Why did they allowed Foo const& foo = fooByValue(); to begin with ? It makes life (somewhat) easier, but introduces potential undefined behavior all over the place. Foo const& fooByReference() { return fooByValue(); // error: returning a reference to a temporary } This is obviously wrong, and … Read more
&std::string(“Hello World”) The problem with this isn’t that std::string(“Hello World”) yields a temporary object. The problem is that the expression std::string(“Hello World”) is an rvalue expression that refers to a temporary object. You cannot take the address of an rvalue because not all rvalues have addresses (and not all rvalues are objects). Consider the following: … Read more
Well, the doc gives the exact reasons when “Using temporary” will appear: Temporary tables can be created under conditions such as these: If there is an ORDER BY clause and a different GROUP BY clause, or if the ORDER BY or GROUP BY contains columns from tables other than the first table in the join … Read more
getString() would return a copy of str (getString() returns by value); It’s right. thus, the copy of str would stay “alive” in main() until main() returns. No, the returned copy is a temporary std::string, which will be destroyed at the end of the statement in which it was created, i.e. before std::cout << cStr << … Read more
This is 90% a duplicate of Return local String as a slice (&str), see that for multiple other solutions. There’s one extra possibility since this is all in one function: You can declare a variable for the String and only set it when you need to allocate. The compiler (obliquely) suggests this: consider using a … Read more
Edit: As j_random_hacker notes, it is possible to force the user to declare a named object in order to take out a lock. However, even if creation of temporaries was somehow banned for your class, then the user could make a similar mistake: // take out a lock: if (m_multiThreaded) { CSingleLock c(&m_criticalSection, TRUE); } … Read more
The standard considers two circumstances under which the lifetime of a temporary is extended: ยง12.2/4 There are two contexts in which temporaries are destroyed at a different point than the end of the fullexpression. The first context is when an expression appears as an initializer for a declarator defining an object. In that context, the … Read more
A modern optimizing compiler should optimize those variables away, for example if we use the following example in godbolt with gcc using the -std=c99 -O3 flags (see it live): #include <stdio.h> void func() { int i = 5; int j = 10; int result = i + j; printf( “%d\n”, result ) ; } it … Read more
This: typedef int& ref; const ref error; Doesn’t do what you think it does. Consider instead: typedef int* pointer; typedef const pointer const_pointer; The type of const_pointer is int* const, not const int *. That is, when you say const T you’re saying “make a type where T is immutable”; so in the previous example, … Read more