How to return a string from a C++ function?

You never give any value to your strings in main so they are empty, and thus obviously the function returns an empty string.

Replace:

string str1, str2, str3;

with:

string str1 = "the dog jumped over the fence";
string str2 = "the";
string str3 = "that";

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Also, you have several problems in your replaceSubstring function:

int index = s1.find(s2, 0);
s1.replace(index, s2.length(), s3);

• std::string::find returns a std::string::size_type (aka. size_t) not an int. Two differences: size_t is unsigned, and it’s not necessarily the same size as an int depending on your platform (eg. on 64 bits Linux or Windows size_t is unsigned 64 bits while int is signed 32 bits).
• What happens if s2 is not part of s1? I’ll leave it up to you to find how to fix that. Hint: std::string::npos 😉