1. 八进制 -> 十进制

#include "stdafx.h"
#include "iostream"
#include "string"
#include <algorithm>

using namespace std;

#define TOLOWER(p) {transform(p.begin(),p.end(),p.begin(),::tolower);}

//八进制 -》十进制
int OctStrToDecStr(string str)
{
	int iDec = 0;
	TOLOWER(str);
	sscanf_s(str.c_str(), "%o", &iDec);
	return iDec;
}

int main()
{
	string str_10 = "173";
	cout << "八进制："<<atoi(str_10.c_str()) << endl;
	int iDec = OctStrToDecStr(str_10);
	cout << "十进制：" << iDec << endl;
	system("pause");
}

结果验证：

2. 十进制 -> 八进制

#include "stdafx.h"
#include "iostream"
#include "string"
#include "vector"

using namespace std;

//十进制 -> 八进制
string DecStrToOctStr(string str)
{
	string str_Oct = "";
	int iDec = atoi(str.c_str());
	int remder = 0;
	vector<int>vec_remder;
	while (iDec / 8 != 0)
	{
		remder = iDec % 8;
		vec_remder.push_back(remder);
		iDec /= 8;
	}
	remder = iDec % 8;
	vec_remder.push_back(remder);
	reverse(vec_remder.begin(), vec_remder.end());
	for (int i = 0; i < vec_remder.size(); i++)
	{
		str_Oct += to_string(vec_remder[i]);
	}

	return str_Oct;
}

int main()
{
	string str_10 = "123";
	cout << "十进制:" << str_10.c_str() << endl;
	string str_Oct = DecStrToOctStr(str_10);
	cout << "八进制:" << str_Oct.c_str() << endl;
	system("pause");
}

结果验证：