Infix to Postfix conversion in C++ using stack. We are assuming that both operators and operands in input will be single character.

InfixToPostfix.cpp

/*
  Infix to postfix conversion in C++ 
  Input Postfix expression must be in a desired format. 
  Operands and operator, both must be single character.
  Only '+'  ,  '-'  , '*', '/' and '$' (for exponentiation)  operators are expected. 
*/
#include<iostream>
#include<stack>
#include<string>

using namespace std;

// Function to convert Infix expression to postfix 
string InfixToPostfix(string expression);

// Function to verify whether an operator has higher precedence over other
int HasHigherPrecedence(char operator1, char operator2);

// Function to verify whether a character is operator symbol or not. 
bool IsOperator(char C);

// Function to verify whether a character is alphanumeric chanaracter (letter or numeric digit) or not. 
bool IsOperand(char C);

int main() 
{
	string expression; 
	cout<<"Enter Infix Expression \n";
	getline(cin,expression);
	string postfix = InfixToPostfix(expression);
	cout<<"Output = "<<postfix<<"\n";
}

// Function to evaluate Postfix expression and return output
string InfixToPostfix(string expression)
{
	// Declaring a Stack from Standard template library in C++. 
	stack<char> S;
	string postfix = ""; // Initialize postfix as empty string.
	for(int i = 0;i< expression.length();i++) {

		// Scanning each character from left. 
		// If character is a delimitter, move on. 
		if(expression[i] == ' ' || expression[i] == ',') continue; 

		// If character is operator, pop two elements from stack, perform operation and push the result back. 
		else if(IsOperator(expression[i])) 
		{
			while(!S.empty() && S.top() != '(' && HasHigherPrecedence(S.top(),expression[i]))
			{
				postfix+= S.top();
				S.pop();
			}
			S.push(expression[i]);
		}
		// Else if character is an operand
		else if(IsOperand(expression[i]))
		{
			postfix +=expression[i];
		}

		else if (expression[i] == '(') 
		{
			S.push(expression[i]);
		}

		else if(expression[i] == ')') 
		{
			while(!S.empty() && S.top() !=  '(') {
				postfix += S.top();
				S.pop();
			}
			S.pop();
		}
	}

	while(!S.empty()) {
		postfix += S.top();
		S.pop();
	}

	return postfix;
}

// Function to verify whether a character is english letter or numeric digit. 
// We are assuming in this solution that operand will be a single character
bool IsOperand(char C) 
{
	if(C >= '0' && C <= '9') return true;
	if(C >= 'a' && C <= 'z') return true;
	if(C >= 'A' && C <= 'Z') return true;
	return false;
}

// Function to verify whether a character is operator symbol or not. 
bool IsOperator(char C)
{
	if(C == '+' || C == '-' || C == '*' || C == '/' || C== '$')
		return true;

	return false;
}

// Function to verify whether an operator is right associative or not. 
int IsRightAssociative(char op)
{
	if(op == '$') return true;
	return false;
}

// Function to get weight of an operator. An operator with higher weight will have higher precedence. 
int GetOperatorWeight(char op)
{
	int weight = -1; 
	switch(op)
	{
	case '+':
	case '-':
		weight = 1;
	case '*':
	case '/':
		weight = 2;
	case '$':
		weight = 3;
	}
	return weight;
}

// Function to perform an operation and return output. 
int HasHigherPrecedence(char op1, char op2)
{
	int op1Weight = GetOperatorWeight(op1);
	int op2Weight = GetOperatorWeight(op2);

	// If operators have equal precedence, return true if they are left associative. 
	// return false, if right associative. 
	// if operator is left-associative, left one should be given priority. 
	if(op1Weight == op2Weight)
	{
		if(IsRightAssociative(op1)) return false;
		else return true;
	}
	return op1Weight > op2Weight ?  true: false;
}

`#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX_SIZE 100

char stack[MAX_SIZE];
int top = -1;

void push(char x)
{
if(top == MAX_SIZE -1) return;
else{
stack[++top] = x;
}
}
char pop()
{
if(top == -1) return -1;
else{
return stack[top--];
}
}

bool isEmpty() {
return top == -1;
}

bool IsOperator(char C){
if (C == '+' || C == '-' || C == '*' || C == '/' || C == '^')
{
return true;
}
return false;
}

bool IsOperand(char C){
if (C >= '0' && C <= '9') return true;
if (C >= 'a' && C <= 'z') return true;
if (C >= 'A' && C <= 'Z') return true;
return false;
}

bool IsRightAssociative(char op){
if(op == '^') return true;
return false;
}

int GetOperatorPriority(char op){
int priority = -1;
switch(op){
case '+':
case '-':
priority = 1;
break;
case '*':
case '/':
priority = 2;
break;
case '^':
priority = 3;
break;
}
return priority;
}

bool HasHigherPrecedence(char op1, char op2){
int op1Priority = GetOperatorPriority(op1);
int op2Priority = GetOperatorPriority(op2);
if(op1Priority == op2Priority){
if (IsRightAssociative(op1)) return false;
else
{
return true;
}
}
return op1Priority > op2Priority;
}

char* InfixToPostfix(char* expression){
int len = strlen(expression);
char* postfix = (char*)malloc(sizeof(char) * (len + 2));
int j = 0;
for(int i = 0; i < len; i++){
if(expression[i] == ' ' || expression[i] == ',') continue;
if(IsOperator(expression[i])){
while(!isEmpty() && stack[top] != '(' && HasHigherPrecedence(stack[top], expression[i])){
postfix[j++] = pop();
}
push(expression[i]);
}

        if(IsOperand(expression[i])){
            postfix[j++] = expression[i];
        }
        else if(expression[i] == '('){
            push(expression[i]);
        }
        else if (expression[i] == ')')
        {
            while (!isEmpty() && stack[top] != '(')
            {
                postfix[j++] = pop(); 
            }
            if (!isEmpty() && stack[top] == '(') {
                pop();
        }
        }

}
while (!isEmpty())
{
postfix[j++] = pop();

    }
    postfix[j] = '\0';
    return postfix;

}

int main(){
char infix[MAX_SIZE] = "a+b*(c^d-e)^(f+g*h)-i";

// Function call
char* postfix = InfixToPostfix(infix);
printf("%s\n", postfix);
free(postfix);
return 0;

}`