c++ / scope of struct

Question

In following code, I think that structure stSameNameButDifferent is local scope definition and so no problem for it. But I got error in run-time. (error : process crash)

Can you explain what's wrong with that code?

test_function.h

#ifndef TEST_FUNC_H_
#define TEST_FUNC_H_
void test_a();
void test_b();

#endif

main.cpp

#include <iostream>
#include "test_function.h"

using namespace std;

int main(int argc, const char** argv)
{
        cout << "testing for struct scope" << endl;
        test_a();
        test_b();
        return 0;
}

test_a.cpp

#include <iostream>
#include <sstream>
#include <cstdint>
#include <list>
#include "test_function.h"

struct stSameNameButDifferent
{
        uint32_t nPlayCode;
        uint32_t nGameID;
        std::string sGameName;
};

void test_a()
{
        std::list<stSameNameButDifferent> lstSt;
        for(int i=0; i<10; ++i)
        {
                stSameNameButDifferent st;
                st.nPlayCode = i;
                st.nGameID = 100+i;
                std::ostringstream osBuf;
                osBuf << "Game_" << i;
                st.sGameName = osBuf.str();
                lstSt.push_back(st);
        }
        for(auto &st : lstSt)
        {
                std::cout << st.nPlayCode << ", " << st.nGameID << ", " << st.sGameName << std::endl;
        }
}

test_b.cpp

#include <iostream>
#include <sstream>
#include <cstdint>
#include <list>
#include "test_function.h"

struct stSameNameButDifferent
{
        uint32_t nPlayCode;
        uint32_t nGameID;
        float    fDiscountRate;
        std::string sGameName;
};

void test_b()
{
        std::list<stSameNameButDifferent> lstSt;
        for(int i=0; i<10; ++i)
        {
                stSameNameButDifferent st;
                st.nPlayCode = i;
                st.nGameID = 1000+i;
                st.fDiscountRate = (float)i/100;
                std::ostringstream osBuf;
                osBuf << "Game_" << i;
                st.sGameName = osBuf.str();
                lstSt.push_back(st);
        }
        for(auto &st : lstSt)
        {
                std::cout << st.nPlayCode << ", " << st.nGameID << ", " << st.sGameName << std::endl;
        }
}

Congratulation, you have met your first non-trivial sized program, now you know why you shouldn't use using namespace std; and would want define your own namespace. — Surt
– Surt, Commented Nov 9, 2015 at 10:53
@heon: And what did your debugger say was the cause? "I got error" and "process crashed" are insufficient problem descriptions. They're not even full sentences. Come on... — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Nov 9, 2015 at 11:38

πάντα ῥεῖ · Accepted Answer · 2015-11-09 10:41:13Z

3

To avoid clashes of the same struct names in multiple translation units, you have to put them in an unnamed namespace like so:

namespace {
    struct stSameNameButDifferent {
        uint32_t nPlayCode;
        uint32_t nGameID;
        std::string sGameName;
    };
}

This will make stSameNameButDifferent only seen privately in the corresponding translation unit (.cpp file).

Otherwise the linker will resolve the symbol with the first one found, hence the errors you see at runtime.

edited Nov 9, 2015 at 10:41

answered Nov 9, 2015 at 10:26

πάντα ῥεῖ

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Comments

ANjaNA · Accepted Answer · 2015-11-09 14:30:07Z

0

you have defined stSameNameButDifferent in global scope, so compiler cannot see and analyze both definitions to same struct and it only take the first one it meet and that's why you are getting an error.

You can use two different namespaces for test_a and test_b, so you will not get any error.

edited Nov 9, 2015 at 14:30

answered Nov 9, 2015 at 10:53

ANjaNA

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1 Comment

Lightness Races in Orbit Over a year ago

"compiler can see two definitions to same struct" You are mistaken.

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