C/C++ – StrayWarrior's Blog

A bug Caused by Implicit Construction of std::string

Recently I met a bug when I was using google protobuf to serialize and deserialize messages.I have a code segment like this:

class MyMessage; // A message class generated by protobuf</code>
void process() {
    MyMessage msg;
    std::string msg_buffer = some_other_func(); // receive binary format "string"
    msg.ParseFromString(msg_buffer.data());
}

class MyMessage; // A message class generated by protobuf</code>

void process() {

MyMessage msg;

std::string msg_buffer = some_other_func(); // receive binary format "string"

msg.ParseFromString(msg_buffer.data());

}

In the code, there is a weird usage of std::string::data() and MyMessage::ParseFromString() API. It’s a mistake due

Be careful with return-value optimization of C++ compilers

Side Effect of Return-value Optimization This “bug” is caused by return-value optimization and affects both GCC and Microsoft C/C++ Compiler. Assume that we have codes like following:

#include <cstdio>
class Complex {
public:
    Complex() : Complex(0.0, 0.0) {}
    Complex(double a, double b) : a(a), b(b) {}
    Complex(const Complex & x) : a(x.a), b(x.b) {
        dummy = 1.f;
    }
    Complex operator * (const Complex & x) {
        return Complex(a * x.a, b * x.b);
    }

    double a, b;
    double dummy;
};

int main() {
    Complex z1(-1, 2);
    Complex z2(3, 5);
    Complex z3 = z1 * z2;
    Complex z4 = z3;
    printf("%f %f %f\n", z3.a, z3.b, z3.dummy);
    printf("%f %f %f\n", z4.a, z4.b, z4.dummy);
    return 0;
}

#include <cstdio>

class Complex {

public:

Complex() : Complex(0.0, 0.0) {}

Complex(double a, double b) : a(a), b(b) {}

Complex(const Complex & x) : a(x.a), b(x.b) {

dummy = 1.f;

}

Complex operator * (const Complex & x) {

return Complex(a * x.a, b * x.b);

}

double a, b;

double dummy;

};

int main() {

Complex z1(-1, 2);

Complex z2(3, 5);

Complex z3 = z1 * z2;

Complex z4 = z3;

printf("%f %f %f\n", z3.a, z3.b, z3.dummy);

printf("%f %f %f\n", z4.a, z4.b, z4.dummy);

return 0;

}

Compile the code with g++ main.cpp -O0 -g -std=c++11. Run the program and the output result is

Link Error When Compiling and Installing Python Package in Windows

It’s usually easy to install a Python package in Windows with pip or other pre-built binaries. However, sometimes we must build the binary from source by ourselves, such as Basemap module, a part of matplotlib-toolkit.Then we may meet some error