Question

My class has a member std::string received;, initialized at an empty string in its constructor, along with a function printReceived that prints the string to cout.

In main(), an instance of the above class is created, and printReceived is called.

Instead of getting an empty string, I get totally unexpected values (but always the same):

• If printReceived is std::cout<<"Received ":<<received<<std::endl;, I get

  Received: eived: as output.

• A string constant present in a function of another class which is not called, if this file is linked.

Where could that come from ? It’s getting me mad… All variables are correctly initialized. I’ve never had this problem before, and I’ve programmed a lot in C++.

Here is a complete minimal example as asked:

CellularTest.cpp

#include "A.h"

#include <iostream>

int main()
{
    A s;

    s.println("AT+CSQ");
    
    return 0;
}

A.cpp

#include "A.h"

A::A()
: received("")
{
}
void A::println(char* s)
{
    received+=s+'\n';
    treatReceived();
}
void A::treatReceived()
{
    std::cout<<"Received: "<<received<<std::endl;
}

A.h

#include <iostream>
#include <string>

class A
{
    public:
        A();
        void println(char* s);
    private:
        std::string received;
        void treatReceived();
};

Makefile

CellularTest: CellularTest.o CellularTest.cpp A.o
    g++ CellularTest.o A.o -o CellularTest

CellularTest.o: CellularTest.cpp

A.o: A.cpp A.h

clean:
    rm *.o
    rm CellularTest

The output I get is:

Received: eived: 

Answer 1

operator+= has lower precedence than operator+. So in println, you’re doing this:

received+=(s+'\n');

Which is like

received+=(s+10);

Which is incrementing the pointer s by 10 bytes and then appending the string pointed to by the resulting char* to received, and the string literal Recieved: is happening to be stored just after the string literal AT+CSQ. So memory might look like AT+CSQ\0Received: \0 and incrementing AT+CSQ by 10 is actually the e in Received:. So there you have it.

Change that to

received+=s;
received+='\n';

Or alternatively

received = received + s + '\n';