smart_pointers_efficiency.md

Efficiency

---

Raw pointer

#include <memory>
#include <vector>

struct Data {
    char tab_[42];
};

int main(void) {
    constexpr unsigned size = 10u * 1000u * 1000u;
    std::vector<Data *> v;
    v.reserve(size);
    for (unsigned i = 0; i < size; ++i) {
        auto p = new Data;
        v.push_back(std::move(p));
    }
    for (auto p: v)
        delete p;
}

---

Unique pointer

#include <memory>
#include <vector>

struct Data {
    char tab_[42];
};

int main(void) {
    constexpr unsigned size = 10u * 1000u * 1000u;
    std::vector<std::unique_ptr<Data>> v;
    v.reserve(size);
    for (unsigned i = 0; i < size; ++i) {
        std::unique_ptr<Data> p{new Data};
        v.push_back(std::move(p));
    }
}

---

Shared pointer

#include <memory>
#include <vector>

struct Data {
    char tab_[42];
};

int main(void) {
    constexpr unsigned size = 10u * 1000u * 1000u;
    std::vector<std::shared_ptr<Data>> v;
    v.reserve(size);
    for (unsigned i = 0; i < size; ++i) {
        std::shared_ptr<Data> p{new Data};
        v.push_back(std::move(p));
    }
}

---

Shared pointer – make_shared

#include <memory>
#include <vector>

struct Data {
    char tab_[42];
};

int main(void) {
    constexpr unsigned size = 10u * 1000u * 1000u;
    std::vector<std::shared_ptr<Data>> v;
    v.reserve(size);
    for (unsigned i = 0; i < size; ++i) {
        auto p = std::make_shared<Data>();
        v.push_back(std::move(p));
    }
}

---

Weak pointer

#include <memory>
#include <vector>

struct Data {
    char tab_[42];
};

int main(void) {
    constexpr unsigned size = 10u * 1000u * 1000u;
    std::vector<std::shared_ptr<Data>> vs;
    std::vector<std::weak_ptr<Data>> vw;
    vs.reserve(size);
    vw.reserve(size);
    for (unsigned i = 0; i < size; ++i) {
        std::shared_ptr<Data> p{new Data};
        std::weak_ptr<Data> w{p};
        vs.push_back(std::move(p));
        vw.push_back(std::move(w));
    }
}

---

Measurements

• gcc-4.8.2
• compilation with –std=c++11 –O3 –DNDEBUG
• measuring with:
  • time (real)
  • htop (mem)
  • valgrind (allocations count)

---

Results

test name	time [s]	allocations	memory [MB]
raw pointer	0.54	10 000 001	686
unique pointer	0.56	10 000 001	686
shared pointer	1.00	20 000 001	1072
make shared	0.76	10 000 001	914
weak pointer	1.28	20 000 002	1222

---

Conclusions

• RAII
  • acquire resource in constructor
  • release resource in destructor
• Rule of 5, Rule of 0
• Smart pointers:
  • std::unique_ptr – primary choice, no overhead, can convert to std::shared_ptr
  • std::shared_ptr – introduces memory and runtime overhead
  • std::weak_ptr – breaking cycles, can convert to/from std::shared_ptr
• Create smart pointers with std::make_shared() and std::make_unique()
• Raw pointer should mean „access only” (no ownership)
• Use reference instead of pointers if possible