rvalue

Is the following code causing undefined behavior? std::map<int, vector<int>> foo() { return ... } BOOST_FOREACH(const int& i, foo()[42]) { std::cout << i << std::endl; } If undefined, What is the good way to fix it? What if I use c++11 range-for loop instead of BOOST_FOREACH? This is, unfortunately, most probably undefined behavior. The problem is that you have two levels here: std::map<...> is an r-value, its lifetime will be expanded until the end of the full-expression std::vector<int>& is an l-value reference (into an object), its lifetime is that of the object. The problem arises because

I have read the SO question here and understood this part of the answer: "But if you bind a temporary to a non-const reference, you can keep passing it around "forever" just to have your manipulation of the object disappear, because somewhere along the way you completely forgot this was a temporary." That is, in the following: #include <iostream> void modifyValue(int& rValue) { rValue++; } int main() { modifyValue(9899); return 0; } If an rvalue could bind to a non-const lvalue reference, then potentially many modifications could be done that would eventually be discarded (since an rvalue is

int x = 12; 12 is said to be integer literal, and therefore can't be used in the LValue. How does the compiler allocate memory to a literals? What is the scope of a literals? Why can't we get its address with an &12 in its scope? OK Bad example in the question. But the question is still valid: Lets try: Foo getFoo() {return Foo();} int func() { getFoo().bar(); // Creates temporary. // before this comment it is also destroyed. // But it lives for the whole expression above // So you can call bar() on it. } int func2() { Foo const& tmp = getFoo(); // Creates temporary. // Does not die here as it

I asked this question: static_assert of const Variable And apparently it comes down to the question does a floating point lvalue get converted to an rvalue for the purposes of comparison? So in this code does an lvalue-to-rvalue conversion occur? const float foo = 13.0F; static_assert(foo > 0.0F, "foo must be greater than 0."); Yes, it is performed. Basically, it's all because 3.0 > 1.2 is a well-formed expression, that contains nothing but prvalues for operands. First, [expr]/9 states (emphasis mine) that Whenever a glvalue expression appears as an operand of an operator that expects a

Why does this code not compile? #include <sstream> void f(std::stringstream) { } int main() { f(std::stringstream{}); } I get this error: error: use of deleted function ‘std::basic_stringstream<char>::basic_stringstream(const std::basic_stringstream<char>&)’ f(std::stringstream{}); ^ If I replace std::stringstream with another type that's noncopyable it works fine. Shouldn't this use stringstream 's move constructor? The missing move constructor for stringstream is a known missing feature in GCC's implementation of the C++ standard library. If I'm reading the comments on that report correctly,

Trying to learn lvalues , rvalues and memory allocation for them. So with a lot of learning materials there is a bit of chaos. An rvalue is a value that needs to exist only in bounds of a expression where it was created (until C++11 at least). So it has an address and block of memory that it occupies. But by definition we cannot get an address of rvalue , because it is a temporary object in contrast to an lvalue . But even before C++11 we were able to get an address of rvalue by returning it from a function and saving it into a const reference type (uh, I guess not an address but a value). So,

I made a class Array which holds an integer array. From the main function, I'm trying to get an element of the array in Array using [ ] as we do for arrays declared in main. I overloaded the operator [ ] as in the following code; the first function returns an lvalue and the second an rvalue (Constructors and other member functions are not shown.) #include <iostream> using namespace std; class Array { public: int& operator[] (const int index) { return a[index]; } int operator[] (const int index) const { return a[index]; } private: int* a; } However, when I try to call those two functions from