object keep his distance around camera c++ opengl

Question

I want to keep the object permanently at a certain distance from the camera. How i can made this? I tried this:

vec3 obj_pos = -cam->Get_CameraPos() ;
obj_pos .z -= 10.0f ;
...
o_modelMatrix = glm::translate(o_modelMatrix, obj_pos);

but it's not working; The object simply stands on the determined position and not moving

Full code of render:

void MasterRenderer::renderPlane() {            
    PlaneShader->useShaderProgram();
    glm::mat4 o_modelMatrix;
    glm::mat4 o_view = cam->Get_ViewMatrix();
    glm::mat4 o_projection = glm::perspective(static_cast<GLfloat>(glm::radians(cam->Get_fov())),
        static_cast<GLfloat>(WIDTH) / static_cast<GLfloat>(HEIGHT), 0.1f, 1000.0f);

    glUniformMatrix4fv(glGetUniformLocation(PlaneShader->ShaderProgramID, "projection"), 1, GL_FALSE, glm::value_ptr(o_projection ));
    glUniformMatrix4fv(glGetUniformLocation(PlaneShader->ShaderProgramID, "view"), 1, GL_FALSE, glm::value_ptr(o_view ));

    vec3 eye_pos =  vec3(o_view [3][0], o_view [3][1], o_view [3][2]); //or cam->getCameraPosition();
    glm::vec3 losDirection = glm::normalize(vec3(0.0f, 0.0f, -1.0f) - eye_pos);

    vec3 obj_pos = eye_pos + losDirection * 1.0f;

    b_modelMatrix = scale(o_modelMatrix, vec3(20.0f));
    b_modelMatrix = glm::translate(b_modelMatrix, obj_pos );

   glUniformMatrix4fv(glGetUniformLocation(PlaneShader->ShaderProgramID, 
   "model"), 1, GL_FALSE, glm::value_ptr(o_modelMatrix));
    ...
    /// draw 

Answer 1

Maybe this is a shot from the hip, but I suppose that you set up a lookat matrix and that you the position of your object is defined in world coordinates.

Commonly a camera is defined by a eye position, at target (center) position and an up vector. The direction in which the camera looks is the line of sight, which is the unit vector from the eye position to the target position.

Calcualte the line of sight:

glm::vec3 cameraPosition ...; // the eye position
glm::vec3 cameraTarget ...;   // the traget (center) posiiton 
glm::vec3 losDirection = glm::normalize( cameraTarget - cameraPosition );

Possibly the camera class knows the direction of view (line of sight), then you can skip this calculation.

If the object is always to be placed a certain distance in front of the camera, the position of the object is the position of the camera plus a distance in the direction of the line of sight:

float distance = ...;
float objectPosition = cameraPosition + losDirection * distance;
glm::mat4 modelPosMat = glm::translate( glm::mat4(1.0f) , objectPosition );

glm::mat4 objectModelMat = ...; // initial model matrix of the object
o_modelMatrix = modelPosMat * objectModelMat;

Note the objectModelMat is the identity matrix if the object has no further transformations glm::mat4(1.0f).