diff --git a/practice/strings/CMakeLists.txt b/practice/strings/CMakeLists.txt
new file mode 100644
index 0000000..672a107
--- /dev/null
+++ b/practice/strings/CMakeLists.txt
@@ -0,0 +1,9 @@
+cmake_minimum_required(VERSION 2.8) # Проверка версии CMake.
+                                                                        # Если версия установленой программы
+                                                                        # старее указаной, произайдёт аварийный выход.
+
+add_executable(main imstring.h main.cpp)		# Создает исполняемый файл с именем main
+                                                                        # из исходника main.cpp
+
+
+set (CMAKE_CXX_FLAGS "-std=c++11")
diff --git a/practice/strings/imstring.cpp b/practice/strings/imstring.cpp
new file mode 100644
index 0000000..9ef1466
--- /dev/null
+++ b/practice/strings/imstring.cpp
@@ -0,0 +1,208 @@
+#include "imstring.h"
+#include <cassert>
+
+imstring::iterator::iterator(std::shared_ptr<buffer> buffer)
+    : root_ (buffer)
+    , current_node_(buffer) {
+    to_next_value();
+    eval_end();
+}
+
+imstring::iterator& imstring::iterator::operator++() {
+    // TODO: implement
+}
+
+imstring::iterator imstring::iterator::operator++(int) {
+    auto copy(*this);
+    this->operator++();
+    return copy;
+}
+
+imstring::buffer::buffer(std::string const& value)
+    : is_concat_(false)
+    , value_(value)
+    , left_(nullptr)
+    , right_(nullptr)
+{}
+
+imstring::buffer::buffer(std::shared_ptr<buffer> left, std::shared_ptr<buffer> right)
+    : is_concat_(true)
+    , value_("")
+    , left_(left)
+    , right_(right)
+{} 
+
+size_t imstring::buffer::size() const {
+    if(is_concat_) {
+        return  (left_ ? left_->size() : 0) + 
+                (right_ ? right_->size() : 0);
+    }
+    
+    return value_.size();
+}
+
+const char* imstring::buffer::c_str() {
+    if(is_concat_) {
+        collapse();
+    }
+
+    assert(!is_concat_);
+    return value_.c_str();
+}
+
+char imstring::buffer::get_at(size_t ix) {
+    if(is_concat_) {
+        auto left_size = left_->size();
+        if(left_->size() < ix) {
+            return left_->get_at(ix);
+        }
+
+        return right_->get_at(ix - left_size);
+    }
+    
+    return value_[ix];
+}
+
+std::string::iterator imstring::buffer::begin() {
+    collapse();
+    return value_.begin();
+}
+
+std::string::iterator imstring::buffer::end() {
+    collapse();
+    return value_.end();
+}
+
+void imstring::buffer::collapse() {
+    if (!is_concat_) return;
+    std::vector<std::shared_ptr<buffer>> primitive_buffers;
+    auto total_size = add_primitive_buffers(primitive_buffers);
+
+    value_.reserve(total_size);
+    value_ = "";
+    for (auto buffer_ptr : primitive_buffers) {
+        value_ += buffer_ptr->value_;
+    }
+
+    right_ = left_ = nullptr;
+    is_concat_ = false;
+}
+
+size_t imstring::buffer::add_primitive_buffers(std::vector<std::shared_ptr<buffer>>& buffers) const {
+    size_t total_size = 0;
+
+    if (left_->is_concat_) {
+        total_size += left_->add_primitive_buffers(buffers);
+    }
+    else {
+        total_size += left_->size();
+        buffers.push_back(left_);
+    }
+
+    if(right_->is_concat_) {
+        total_size += right_->add_primitive_buffers(buffers);
+    }
+    else {
+        total_size += right_->size();
+        buffers.push_back(right_);
+    }
+
+    return total_size;
+}
+
+imstring::iterator::iterator()
+    : root_(nullptr)
+    , current_node_(nullptr)
+    , current_pos_(std::string::iterator())
+    , end_pos_(std::string::iterator())
+    , stack_()
+{}
+
+bool imstring::iterator::to_next_value() {
+    if (current_pos_ == end_pos_) {
+        return false;
+    }
+
+    if (current_node_->is_concat_) {
+        while (current_node_->is_concat_) {
+            stack_.push(current_node_);
+            current_node_ = current_node_->left_;
+        }
+
+        current_pos_ = current_node_->value_.begin();
+        current_end_ = current_node_->value_.end();
+        return true;
+    }
+
+
+
+    // TODO: Complete method.
+}
+
+void imstring::iterator::eval_end() {
+    auto node = root_;
+    while(!node->is_concat_) {
+        node = node->right_;
+    }
+
+    end_pos_ = node->value_.end();
+}
+
+imstring::imstring()
+    : shared_buffer_(std::make_shared<buffer>(""))
+{}
+
+imstring::imstring(const char* c_str)
+    : shared_buffer_(std::make_shared<buffer>(c_str))
+{}
+
+imstring::imstring(imstring const& other)
+    : shared_buffer_(other.shared_buffer_)
+{}
+
+imstring::imstring(imstring&& other)
+    : shared_buffer_(std::move(other.shared_buffer_))
+{}
+
+size_t imstring::size() const {
+    return shared_buffer_->size();
+}
+
+const char* imstring::c_str() const {
+    return shared_buffer_->c_str();
+}
+
+imstring& imstring::operator+=(imstring const& other) {
+    shared_buffer_ = std::make_shared<buffer>(shared_buffer_, other.shared_buffer_);
+    return *this;
+}
+
+char imstring::operator[](int ix) const {
+    return shared_buffer_->get_at(ix);
+}
+
+std::string::iterator imstring::begin() const {
+    return shared_buffer_->begin();
+}
+
+std::string::iterator imstring::end() const {
+    return shared_buffer_->end();
+}
+
+imstring::iterator imstring::_begin() const {
+    return iterator(shared_buffer_);
+}
+
+imstring::iterator imstring::_end() const {
+    return iterator();
+}
+
+imstring operator+(imstring left, imstring const& right) {
+    return left += right;
+}
+
+std::ostream& operator<<(std::ostream& stream, imstring const& imstring) {
+    // TODO: make more lazy
+    stream << imstring.c_str();
+    return stream;
+}
diff --git a/practice/strings/imstring.h b/practice/strings/imstring.h
new file mode 100644
index 0000000..4fb1770
--- /dev/null
+++ b/practice/strings/imstring.h
@@ -0,0 +1,99 @@
+#pragma once
+#include <string>
+#include <memory>
+#include <vector>
+#include <stack>
+
+struct imstring;
+
+struct imstring {
+private:
+    struct iterator;
+    struct buffer {
+        buffer(std::string const& value);
+
+        buffer(std::shared_ptr<buffer> left, std::shared_ptr<buffer> right);
+
+        size_t size() const;
+        const char* c_str();
+        char get_at(size_t ix);
+
+        std::string::iterator begin();
+        std::string::iterator end();
+
+        friend struct iterator;
+    private:
+        void collapse();
+        size_t add_primitive_buffers(std::vector<std::shared_ptr<buffer>>& buffers) const;
+        
+        bool is_concat_;
+        std::string value_;
+        std::shared_ptr<buffer> left_;
+        std::shared_ptr<buffer> right_;
+    }; // buffer
+
+public:
+    struct iterator : std::iterator<std::bidirectional_iterator_tag, char> {
+        explicit iterator(std::shared_ptr<buffer> buffer);
+
+        // all categories
+        iterator(iterator const& other) = default;
+        iterator& operator=(iterator const& other) = default;
+
+        iterator& operator++();
+        iterator  operator++(int);
+
+        // input iterator
+        // TODO: move out-scope
+        bool operator==(iterator const& other);
+        bool operator!=(iterator const& other);
+
+        char operator*();
+        char* operator->();
+
+        // forward iterator
+        iterator();
+
+        // bidirectional iterator
+        iterator& operator--();
+        iterator  operator--(int);
+
+    private:
+        bool to_next_value();
+        bool to_prev_value();
+        void eval_end();
+        std::shared_ptr<buffer> root_;
+
+        std::shared_ptr<buffer> current_node_;
+        std::string::iterator current_pos_;
+        std::string::iterator current_end_;
+        std::string::iterator end_pos_;
+
+        std::stack<std::shared_ptr<buffer>> stack_;
+    }; // iterator
+
+public:
+    imstring();
+    imstring(const char*);
+    imstring(imstring const& other);
+    imstring(imstring && other);
+    imstring& operator=(imstring const& other) = delete;
+
+    size_t size() const;
+    const char* c_str() const;
+
+    imstring& operator+=(imstring const& other);
+    char operator[](int ix) const;
+
+    std::string::iterator begin() const;
+    std::string::iterator end() const;
+
+    iterator _begin() const;
+    iterator _end() const;
+
+private:
+    std::shared_ptr<buffer> shared_buffer_;
+};
+
+imstring operator+(imstring left, imstring const& right);
+std::ostream& operator<<(std::ostream& ostream, imstring const& imstring);
diff --git a/practice/strings/main.cpp b/practice/strings/main.cpp
new file mode 100644
index 0000000..369008e
--- /dev/null
+++ b/practice/strings/main.cpp
@@ -0,0 +1,62 @@
+#include "imstring.h"
+#include <string>
+#include <cassert>
+#include <cstring>
+#include <iterator>
+#include <algorithm>
+#include <iostream>
+
+template<class STR1, class STR2>
+void test_strings_equal(STR1& str1, STR2 &str2)
+{
+    assert(str1.size() == str2.size());
+    assert(std::equal(str1.begin(), str1.end(), str2.begin()));
+    assert(std::distance(str1.begin(), str1.end()) ==
+            std::distance(str2.begin(), str2.end()));
+    // call operator << before we force strings to be not lazy
+    std::cout << str1 << std::endl << str2 << std::endl;
+    assert(strcmp(str1.c_str(), str2.c_str()) == 0);
+}
+
+void my_tests() {
+    std::string str("abc");
+
+    auto it = str.begin();
+    ++it;
+    assert(*it == 'b');
+    --it;
+    assert(*it == 'a');
+}
+
+int main()
+{
+    std::string str1("foo bar buzz");
+
+    // Test creation from implicit c string
+    imstring imstr1 = str1.c_str();
+    test_strings_equal(str1, imstr1);
+    test_strings_equal(imstr1, str1);
+
+    // Test copying of shared immutable string buffer
+    imstring imstr1_cp(imstr1);
+    test_strings_equal(str1, imstr1);
+    test_strings_equal(imstr1, imstr1_cp);
+    assert(imstr1.begin() == imstr1.begin());
+    assert(imstr1.end() == imstr1.end());
+    // check laziness
+    assert(&(*imstr1.begin()) == &(*imstr1_cp.begin()));
+
+    // Test lazy concatenation
+    imstring imstr_concat1 = imstr1 + imstr1_cp;
+    imstring imstr_concat2 = imstr_concat1 + imstr_concat1 +
+        imstr1 + imstr1_cp + imstr_concat1;
+    std::string str_concat1 = str1 + str1;
+    std::string str_concat2 = str_concat1 + str_concat1 +
+        str1 + str1 + str_concat1;
+    test_strings_equal(str_concat2, imstr_concat2);
+    test_strings_equal(imstr_concat2, str_concat2);
+
+    my_tests();
+
+    return 0;
+}