3 ms·

I read it as the author almost never uses the uninitialized data and when they do need a zeroed array it's simple to do so why pay the cost to initialize the el

• cf141q5325 8y ago
  There is also the approach to just copy objects existing objects to safe yourself the cost of running the constructor in order to shorten your startup time.
• kllrnohj 8y ago
  But you can easily avoid that cost on std::vector, too. std::vector doesn't default-initialize unless you ask it to via resize(). reserve() doesn't initialize, though, it just allocates. Take this toy example: #include <vector> #include <cstdio> struct MyFoo { MyFoo() { printf("default ctor\n"); } MyFoo(int i) { printf("Initialized with %d\n", i); } MyFoo(const MyFoo& foo) {} }; int main() { std::vector<MyFoo> myvec; myvec.reserve(100); printf("resize(2)\n"); myvec.resize(2); printf("emplace_back()\n"); myvec.emplace_back(); printf("emplace_back(1)\n"); myvec.emplace_back(1); printf("push_back(MyFoo{1})\n"); myvec.push_back(MyFoo{2}); return 0; } It will print: resize(2) default ctor default ctor emplace_back() default ctor emplace_back(1) Initialized with 1 push_back(MyFoo{1}) Initialized with 2 Only the resize() call did a default initialization "silently", nothing else did.
  • thestoicattack 8y ago
    The tradeoff is that emplace_back and push_back need to check the capacity (even though they won't reallocate because of reserve). Whereas with resize, assignments into the initialized area need no checks at all.