Application crash when using library from the program compiled with make - c++

Question

I'm making scientific calculator in command line in c++ for my usage and also for practice. I have a problem with compiling it using cmake with mingw on windows. These are my source files:
main.ccp
#include <iostream>
#include <string>
#include "ExpressionCalculations/ExpressionParser.h"
int main()
{
std::string humanReadableExpression;
std::cout<<"Enter expression\n";
std::getline(std::cin, humanReadableExpression);
std::cout<<humanReadableExpression;
ExpressionCalculations::ExpressionParser parser;
auto&& expression = parser.GenerateRpnExpression(humanReadableExpression);
return 0;
}
ExpressionParser.h
#pragma once
#include <memory>
#include <stack>
#include <string>
#include <unordered_map>
namespace ExpressionCalculations
{
class ExpressionParser
{
public:
std::unique_ptr<std::string> GenerateRpnExpression(std::string &humanReadableExpression);
private:
// other code
};
}
ExpressionParser.cpp
#include <memory>
#include <stack>
#include <string>
#include <unordered_map>
#include <iostream>
#include "ExpressionParser.h"
namespace ExpressionCalculations
{
std::unique_ptr<std::string> ExpressionParser::GenerateRpnExpression(
std::string& humanReadableExpression)
{
std::unique_ptr<std::string> rpnExpression;
*rpnExpression="3456";
return rpnExpression;
}
These are cmake files
main CMakeLists.txt
cmake_minimum_required (VERSION 2.8)
project (ScientificCalculator_exe)
add_subdirectory(ExpressionCalculations)
add_executable(ScientificCalculator main.cpp)
target_link_libraries(ScientificCalculator ExpressionCalculations)
module CMakeList.txt
set(calculators ExpressionParser.h ExpressionParser.cpp)
add_library(ExpressionCalculations ${calculators})
When I run it , I can see Enter expression and pass input. Then I get Segmentation fault. However when I remove declaration of ExpressionParser and auto&& expression the string is shown, a string can be inputted and shown in the command. I checked configuration question multiple directories under cmake, https://cmake.org/cmake-tutorial/ and https://www.codeproject.com/Articles/1181455/A-CMake-tutorial-for-Visual-Cplusplus-developers but it seems that I correctly made cmake files. I have no idea why it doesn't work. I use the latest mingw64 on windows with default make compilation parameters.

Answer 1

From the cppreference page on unique_ptr:
The class satisfies the requirements of MoveConstructible and MoveAssignable, but not the requirements of either CopyConstructible or CopyAssignable.
In your ExpressionParser::GenerateRpnExpression function you are attempting to copy the unique_ptr out of the function when you should be moving it. Try return std::move(rpnExpression)

Answer 2

After debugging program compiled with just g++ I found the problem. It was misunderstanding of unique_ptr' default constructor behaviour. I thought it would initialize std::string but after reading doc and checking it it does not initialize object and generates nullptr. Then I looked into Scott Myers's Modern Effective C++ how to initialize the unique_ptr. Instead of std::unique_ptr<std::string> rpnExpression; I used auto rpnExpression = std::make_unique<std::string>();. It works like charm. I checked compiling through cmake and there were not any problems.